Jeff:
Re-reading your orig post, I see that we all have offered several options, while still ignoring your initial, 3 short, very specific questions, so I will try to address those below.
Your Initial Questions:
1) Those with similar coach to ours ('05 Montery Ventura II), is there supposed to be a heat register in the toilet bath?
2) If not, has anyone put one in?
3) And what's involved?
1) As far as I know, only by owning our 05 Monterey Ventura IV (4 slide) for 11 years (BTW: we are the second owners of this coach, purchased in 2006), the answer is a SORT OF a NO; as in no specific Hydro hot floor heat register was factory installed in the enclosed toilet bath area, or what Beaver called in their brochure as the "Private Bath". That is assuming that your coach, being a Monterey Ventura II (2 slide) had the Hydro hot installed as a $7500 upgrade option. On any 2005 Monterey Ventura III (3 slide) or IV (4 slide), the Hydro Hot was a unconditional forced model buy option, so those of us with 3 or 4 slides models might think our Hydro units Hot were included in the "base" model as standard equip - I actually did till just today so I stand corrected.
Note 1: If your coach does not have a Hydro Hot as an installed option, then I do not know what heat registers, if any, would have been installed where, using their 42K BTU electronic furnace (assume propane fired). I have never seen/been inside a non-Hydro Hot-equiped Beaver coach, but I'm sure there are many out there.
Note 2: The standard feature list from the 2005 brochure for all Ventura models is (2) Duo-Therm 15M BTU ducted ceiling AC & HEAT PUMPS, and since there is a dedicated ceiling duct vent in the toilet bath enclosed area, so then YES, there is SORT OF a heat register/source for that enclosed area. Bu I do not believe that is what you were initially asking for in your question #1.
2) I do not remember anyone on the forums ever talking about installing a new Hydro hot heat register in a Ventura model enclosed toilet bath area, or other model for that matter, but I could easily have missed those posts. Others might chime one as to when/where those posts might have been. I have seen posts of others wanting to swap out the noisy Monaco-chosen heat exchangers/blowers with quieter ones in past years, but that was 1-for-1 swap out, not a new add.
I can say I spent more than a few hrs crawling around our Ventura IV basement & enclosed cabinet areas looking for ways/routs to do that exact change you asked about (adding a new exchanger in that specific area) and developed a list of pros/cons & should-costs for each option I came up with. Ultimately I, like others above, decided it was either too hard or too costly to try do add a new HydroHot toe kick exchanger, since there other easier/cheaper/etc methods to get heat into the enclosed toilet bath room.
3) I am more than willing to share my possible installation options / issues with you (e.g.: PEX runs, where to tee/tap off existing into the heat loops, running new 12V blower elect, can the Hydro Hot model HHE-200-09E in the Ventura line support another added heat exchanger without lost of efficiency, etc) if you decide to go down that path of a new Hydro Hot toe kick register in your Ventura II toilet bath enclosed room.
In the end, I choose a completely different route to get heat into the toilet bath area, without a new Hydro hot toe kick register install.
Hope this helps answer your initial 3 questions from this post.