it should already be possible to lauch every available interceptor from every base (that has weapon(s) and ammo) to fight a ufo - please try the latest beta installers (but be warned, that you can't use your 'old' savegames)
Okay -- I finally built up to a point where I can try this in 132xx. Unfortunately, whenever I double-click on a UFO in flight, I still only get the option to send out the dropship from my original base (PHALANX HQ, renamed "Base Alpha", placed somewhere around Germany). Even when a UFO flies into the radar range of base number three ("Base Gamma", about in the middle of Brazil), even though that base has its own armed dropship with soldier complement (and it's done arming -- I checked!), I will only have the option to launch my Base Alpha dropship to intercept.
And I never get the chance to launch my crewed and armed interceptors in any case.
If and when the UFO is shot down or lands, however, I can send any dropship I own to the site.
While I'm on this, I've noticed some other weirdnesses. It used to be that three radars would cover roughly half the planet -- now they barely cover an area as large as the United States (with the exception of Base Alpha, which seems to be blind no matter how many radars I add). And there seem to be a lot more UFOs flying around much earlier in the game. All of this may be normal, though, for all I know.
Thanks for your patience.