TB-5 & TB-15 are similar in some ways, and different in others. Custom 8 has a bit less treble bite, but Alt-8's upper mids come across as a bit less dominant to some ears. A blend ought to wind up with a bit of both, but with an airier feel, rather than a focused present bite. Though sometimes hybrids surprise, and it could wind up favoring the hotter & brighter Alt-8 coil, in which case I'd probably try A6 or Unoriented A5 next, though the last isn't likely to wind up where you were aiming, it might be a very interesting pickup for Custom UOA5 fans...
I don't think I've heard anyone talk about doing one. I'm very curious, please let us know how it turns out if you try it!