Just did a few calculations:

Hex opening: 1,25" - no larger TB can be fitted through the TB anchor.
1,5" spline bar ends have enough meat to be machined down to a hex of 1,25" (distance between opposite flat sides not corners).

So if you get splined bars with 1,5" ends and a diameter no larger than 1,25" you could turn them into regular hex bars if you have access to the necessary machining equipment.

Unfortunately most bars I keep finding in the right diameter are only 37.5" long...