Yes, I bought the one with the 5240110 form factor, but while trying to carve my parts unit frame I found I need better tools. Meanwhile the battery from the parts unit seems to be in excellent condition so I'll use that for a while. Anyway, I've found 1.25 JST connectors work but they're a tad too small, I'll try 1.5 instead -- they just look perfect. You can get a 50x package of either for $5 so no big deal.
Anyway, since I replaced the destroyed reset control of my working unit with an extremely simple wire connection and it works like a charm (I need to post a picture in that thread), I think a better multiple battery solution is a three way switch that connects either one of two batteries or shorts the reset. You'd have to restart the device while switching from one battery to the other and take two charge periods, but you'll get 2x batteries while keeping them independent to avoid other issues. I've bought one 43HH1U1 switch; it's a bit bulky but otherwise perfect according to its data sheet. Also it would require only extremely simple wiring, no soldering and just a bit of dremeling.
I'll post my results when I get the appropriate tools and some spare time.