The 3/4 measures are somewhat interspersed throughout Uncle John’s Band. Just going by the studio version for this answer and by the Garcia/Hunter Songbook. There is an 8 measure 4/4 intro. Then the verse starts, The verse has two measures of 4/4, followed by 1 of 3/4 and then another 4/4, i.e Joe’s suggestion of 15/4. This repeats 1x, followed by 10 measures of 4/4. A verse has 2 - 15/4 sections, followed by the 10 measures of 4/4. This could be accomplished with 2 BB Main parts. That whole thing repeats, so 2 more B main parts, Then you are into the prechorus and chorus, 16 measures of 4/4, followed by an 8 measures interlude. That whole big section repeats, 2 verses, chorus and prechorus, but the interlude then changes to, first 12 bars of alternating 4/4 and 3/4, a possibility for 7/4 section. But, on the 4th time through, the final 3/4 bar is another 4/4 bar then followed by a 3/4 bar and 7 bars of 4/4. So that whole bit could be a 7/4 section, followed by one 11/4 section and then 7 bars of 4/4. This takes you to the outro chorus, which is 8 bars of 4/4 twice, and then the ending, alternating 4/4 and 3/4 (another 7/4 bit) until the hold on the end.
So, can it be done with BB parts? Sure. The key thing is to figure out where you would want your jam section to fall. You could, in effect, have a single long one press like part leading up to your open jam part, followed by another single long one press part to wrap the song up. you might need to play around with time signatures to get something to fit. There do seem to by an even number of 3/4 measures in sections, so this could lead to a possibility of writing the whole thing in 2/4.
I’m positive a have a version posted that could be torn apart and cut up into sections, if anyone wants to give it a try.