If a frames bottom bracket was designed specifically for a THM / Zipp Vuma crankset, are other BB30 crankset options usable?

yes.

No.

THM and Zipp are not real BB30 cranks. Only a BB30/PF30 bottom bracket shell can take a real BB30 crank.

What frame are you concerned about?

Zipp VumaQuad, Rotor 3d, Lightning, Quarq "BB30" cranks have 30mm diameter spindles that are longer than the BB30 spec. That allows them to fit in press fit bearing frames that position their 30mm ID bearings wider than the BB30 86mm (such as BBright), or in BSA frames with external "BSA30" BB cups that hold 30mm ID BB30 bearings instead of 24mm or 24/22mm ID Shimano or GXP bearings. Of course they can fit in BB30 frames too, with spacers.

Specialized and Cannondale are two true BB30 cranks. Their short spindles will only fit in BB30 or PF30 frames.

I was considering purchasing a used boutique brand frame built specifically for THM cranks but didn't want to be limited to choice of cranksets. The bearings are pressed into cups mounted outboard of the BB shell, since true BB30s have their bearings inboard of the BB shell I was wondering what's compatible because of spindle width difference...or if anything is compatible without modifications that would affect performance.

Bearings in cups mounted outside the BB shell could be a threaded bottom bracket shell with BSA30 cups. Or are the cups pressed in to the frame rather than screwed in?

I'm having a hard time imagining how a frame could be built specifically for THM cranks since those cranks(assuming clavicula or clavicula M) will fit into just about every frame that's available. Is there any info on what makes this frame so unique?

The frame in question is likely the Parlee on E-bay. Ironically enough, the guy had the frame and cranks on there for only a abit more than the frame alone a few weeks back. Great looking frame (if it's the one you're looking at) but that dedicated BB might eventually make you question your decision. Especially whereas BB standards seem to change every two years. Seems if I were selling a frame with a dedicated BB and had the cranks, I'd be selling the cranks with it.

Get the serial # from the seller, and contact Parlee to get a proper answer. Fwiw Those are outboard bearings. The question is why would the BB be limited to only THM.

Boolinwall, you're correct.That is the frame I was considering. I asked the seller if the BB was threaded and he stated no threads.

The cups would have been bonded in during manufacturing. Great design really, it would eliminate a lot of potential creaking and save a few grams (the creaking is the real selling point though) Sad really, because that might be the hottest frame on E-bay right now. Or anywhere else for that matter.

Any 386evo crank from FSA should fit, as well as the Rotor 3d+ and 3dforged, and most 24mm spindle cranks with adapters. So that's most cranks. Pure bb30 is out.