Using a shared output seems to me to be the easiest and cheapest solution, especially if the majority of your content is at one sample rate. Unless bitperfect output is very important to you and all of your zones are used for critical listening, I think that's the 95% solution.
If you're like most people I know, you have one setup that you use for "serious hi-fi" and your other zones are mostly used for background music/casual listening, etc. If you're in that camp you could probably set three of your zones to a shared mode and never notice. And WASAPI in shared mode isn't bad, as long as you do any necessary resampling in JRiver. In some cases, I'm not confident that the differences between music at its native sample rate and music resampled by JRiver are even audible. I certainly can't consistently hear the difference in a blind ABX (conceding that I may just have a tin ear).
If bitperfect is important to you (or you can reliably hear the difference between resampled content and original content), for your "serious hi fi" zone, you could setup a shared zone and a separate "hi fi" exclusive access zone, so you could still optionally listen at native sample rates (recognizing that you might miss the doorbell in those intervals).
If all your zones are used for critical listening, then I agree that it might well be worth investing in separate hardware. I'll second the recommendation for separate speaker(s). Tiny speakers are very, very cheap, and setting up dedicated paging speakers will probably be cheaper than even the cheapest mixer(s) (that would actually meet your needs). For example, parts express sells dozens of mini-drivers that are less than $5 a piece. If you look for buyouts you can find 2" full-range drivers for $1 or $2 a unit. If you just want a piezo buzzer (i.e. just ringing, no talking), you can get down under a $1. So you could probably rig up separate paging speakers for $10 plus the costs of wiring and screws. You can mount them in the wall with a hole saw for free, or build enclosures for cheap (although you'll probably spend more on the enclosures than you did on the parts).
If I were in your shoes, though, and really wanted seamless paging, I would just use a shared mode output. 99% of my music is 44.1KHz: I would put every zone in non-exclusive WASAPI, set my windows mixer to 44.1KHz, setup resampling in JRiver for the odd file at a different sample rate, and never look back.