I don't believe that the concept of HSR is compatible to that of party trains.
But just a thought here.
Maybe if a third party wants to sporadically run a party train, Brightline West might be more amenable to letting them use their ROW than UPRR (or whoever it is that that owns the current ROW there). People having a party don't necessarily want to arrive as quickly as possible, and thus conventional equipment moving at normal speeds could be used if it can be rendered compatible to the safety standards of HSR (which seems to be possible on NEC for example). Suitably positioned sidings could be used to get the party train out of the way and allow high speed trains to overtake as necessary.
Actually, IIRC the French occasionally did an overnight "party train" with their TGVs (Paris-Nice IIRC).
Another thought here: How one defines "party train" or "party car" is a good question. There are some conceptions which would be incompatible and others which would be quite compatible.
For example, if you're just designating a car as a "party car" and giving them a fairly full bar to play with (and perhaps some big screen TVs on the walls at each end of the car), that seems doable. You could even go cashless-but-paid if you basically preauthorize somebody for up to $X (where $X is a fairly high amount - say, $150-200) and just bill them by scanning their tickets (since most tickets - physical or digital - are scannable with either a bar code or a QR code).
Even a "standing bar" concept wouldn't be too hard to envision - that's just an Amcafe with most of the seats pulled out. A situation where you have the following configuration for the train could enable that:
LOCO-COACH-COACH-COACH-COACH-COACH-CAFE/BAR-PARTY-PARTY-LOCO
["PARTY" is basically where the party-class pax would be seated.]
Another version would be something like this (based on the long Eurostar sets):
LOCO-PARTY-PARTY-CAFE/BAR-COACH-COACH-COACH-COACH-COACH-COACH-CAFE/BAR-BUSINESS-BUSINESS-LOCO
Both trains aren't married to car counts; it's just a general concept.
Essentially, you have two "premium class" areas with different classes of service aimed in different directions. You'd probably need 3-4 OBS for such a train: A "standard" cafe attendant for each cafe (though you could plausibly only use one cafe), an extra bar server for the Party Class Bar, and possibly an additional attendant for the other premium cabin for at-seat service.