That is one way to extend the encoding from 4-byte chunks to arbitrary byte lengths. Another is to encode 4k+i bytes as 5k+1+i base-85 characters, for 0<i<4. T

That is one way to extend the encoding from 4-byte chunks to arbitrary byte lengths. Another is to encode 4k+i bytes as 5k+1+i base-85 characters, for 0<i<4. That way the encoding length immediately determines the input length. And there's again plenty of space since 85^{i+1} > 256^i for i < 4. This leaves encoding lengths of 5k+1 unused. These could be used to support arbitrary bit lengths, i.e. for encoding an input of 4(k-1) bytes + i bits, with 0<i<32. Set the final byte to i, and let the preceding 5 base-85 characters encode the i bits with 0 padding.