Anyone ever replaced their fixed tune-o-matic bridge with a trem? I was looking at the available models and it seems like they're either super-clunky, like the Bigsbys or stetbars, look completely out of place on the guitar like the Floyd Rose FRX, leave some dumb holes behind like the one from Schaller... apparently Duesenberg has the Les Trem II, which seems like it's a relatively simple and ideal design, but it's hard to find info on it. Not really actually thinking of installing one, but I'm surprised there isn't more choice...
You shouldn't be surprised there aren't more choices; guitars with trems are very easy to find. Most folks who have TOM bridges really didn't want a trem in the first place, or they'll spend the money to have a proper trem routed and/or fitted. The tack-ons are generally less than satisfactory (depending on your taste levels) and the market for them is small.
They all suck in one way or another, basically. Turns out you need a bunch of room for springs and levers for a decent trem, so as you noticed it's not possible to stuff all those things in the footprint of a TOM. Just not enough room. You're right that there aren't a lot of choices, but if you've played the ones that are out there you might understand why: it appears it's not possible to make a competitive, full-range drop-in trem for a TOM. All the existing designs make some pretty serious tradeoffs. As mentioned above, if you want the full experience you really need a new guitar or a proper, full-on routed trem installation.

The Bigsby is my favorite, but I only use a little wobble. It is clunky and it is an ancient design, and it's good for "true" vibrato (smooth sharp and flat) but not for anything beyond that. It needs some extra attention but it can be made to be stable. Basically you need a graphite nut, a roller bridge, and a Vibramate with a USA Bigsby, which is a rather expensive shopping list for just a bit of vibrato.

The Stetsbar is also clunky but it's a little more modern. I had the same problem with the Stetsbar and the Les Trem which is that they feel flimsy, cheap, and don't set up super well. It makes some sense - there's only room for a very short spring, so the more range you try to wring out of the trem, the softer the spring has to be to allow you to do it. A strat can have really long springs, so you can have a stiff tension and just pull the string farther, but with the tiny little springs wedged in these aftermarket TOM units, the only options are to have very little effective range (Bigsby) and/or a mushy spring (Stetsbar, Les Trem).

I haven't used the FRX but I hear it has some of the same issues. It's just a tough nut to crack. Maybe someone will do it eventually but my feeling is that if it were possible to get a good trem in that space, someone would have done it already. All of the really top-notch trem systems use a lot of routed space in the body. There's a good reason for that, and apparently no substitute for it either.
Thanks for the feedback. I've been practicing some Chet Atkins songs lately and with the TOM I can only use my fingers where he uses his Bigsby, but it's not the same thing... but it's not a big deal I suppose...