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Whitish material did not seem to be part of the fly. Fly was large and robust and still capable of buzzing but apparently not capable of flying, found perched on a twig face down with its abdomen raised up. Possible fungal material adhered readily to other surfaces when disturbed - see splatters on side of bug box in image 2. It is possible that this Bot Fly was also infected with the Fly Death Fungus, which is known to occur nearby and does cause flies to lose mobility and assume the posture described above.

Our botanical hiking group was intrigued by the many conspicuously yellowed-but-persistent leaves on this "late deciduous" oak. (Perhaps "partially deciduous" would be a better phrase here...as there were also many green leaves still attached on this nearly mid-winter Feb 1st date.) There were many other deciduous oaks in the local area (presumably many were Q. lobata, and some Q. douglasii)...but they had all completely dropped their leaves by this time.

The intermediate evergreen/deciduous behavior here, together with leaf characters (i.e. lobing, somewhat shiny upper surface, paler lower surface, vestiture, etc.) suggested this was a hybrid of a deciduous and an evergreen pair of parent species in the "white oak group" (Quercus sect. Quercus). We were thinking perhaps Q. douglasii and one of the local scrub oaks (i.e. Q. berberidifolia or Q. durata). Later, @joergmlpts referred me to this nearby iNat obs...which appears as if it may actually be the same exact tree we saw. And there's also this similar-looking iNat obs 38209307 from very nearby. Both those observations were placed as Q. douglasii...but the observers indicated they suspected they were hybrids of Q. douglasii crossed with a scrub oak.

On researching things in the days after our visit, I found a likely candidate: Quercus x subconvexa. It was described by John Tucker in 1953 as a hybrid of Q. garryana (Oregon Oak) and Q. durata (Leather Oak)...both of which are vouchered from the area (indeed Tucker found individuals of each nearby). The agreement is excellent(!) between this oak and the many details of Tucker's description & extended discussion of Q. x subconvexa. Of particular note, diagnostically, are: 1) the lobing pattern of the leaves...each lobe with a short, sharp mucro at its tip; 2) the vestiture of the leaves (uniformly-densely distributed long-rayed stellate hairs abaxially vs. more isolated & widely-scattered tiny clumps of tightly-tufted short-rayed hairs adaxially); as well as the partial deciduousness and paucity/apparent lack of acorns.

And, intriguingly, this site appears to by quite near (and perhaps is! ) the type locality for Q. x subconvexa...which was described as a "north slope, elev. 1300 feet" (compare with topo map link here) and "approximately 5 miles north-northeast of Gilroy". I checked on Google Earth and the spot here fits the bill extremely well. Note also that all Tucker's many vouchers listed here are from the same locality (though the coords given there, 37.072628 -121.532169, appear a bit off...as they indicate a point in an open grassy area on a southwest-facing slope, about 750' to the south of the location of this observation). As seen in the 7th photo here, this observation is just north of two park benches and an interpretive sign along the Mummy Mountain Trail.

The strong fit between critical features of this oak with those discernible in photos accompanying 19 of Tucker's vouchers at the preceding SEINet link (see also here)...as well as the agreement with a preponderance of the many details in the nicely written paper Tucker(1953)... have me fairly-well convinced of the ID here.

---Comments on Individual Photos of the Series---

2nd Photo: Thumbnail is 16 mm wide. Together with Photos 8-10 here, one gets a sense of the relatively large size of the leaves here...presumably reflective of parentage from the relatively large-leaved species Q. garryana.

6th Photo: Tree-like habitus of Q. x subconvexa at center of photo (it looks like a 2nd smaller individual may also be present to the right).

8th Photo: Abaxial side is shown in the duller attached "twig leaves" at upper right center & at lower right ; and also for the separated leaf at lower left...all other leaves here exhibit the shinier adaxial side.

9th Photo: View of adaxial (upper) side, with mm scale.

10th Photo: View of abaxial (lower) side, with mm scale.

11th Photo: Pale abaxial side at leaf vs. shinier adaxial side at right.

12th & 13th photos show detail of abaxial sides. Though it's difficult to make out in the photos (but much better discerned under a well-lit stereo-view dissecting microscope), the abaxial hairs here are somewhat dense and "stellate". In particular, most these stellate hairs had 5-7 relatively-long & straight rays...each ray centrally-attached to the base of the (compound) hair and radiating outward & slightly upward. The attachment of the rays was visually very subtle to perceive under a stereo-view dissecting scope at 40X magnification and could easily be overlooked. I wasn't able to discern it with a hand lens.

I saw a few of these wasps on the trail, some of them were carrying and burying Shield-backed Katydids, like this one -
https://www.inaturalist.org/observations/30787537

I took much better photos (I think) with my real camera of the whole process, but unfortunately I formatted all photos from that day :(