For .mptm format, leverage an undefined channel effect (perhaps S92/S93) that slides up/down the tempo. This would allow one pattern to be used again, with a change in tempo in the same way volume can be altered by sliding value (by calling DF1 for example, then using D00).
If you use set tempo (Txx) with a value of less that 32 (hex 20) you can do this. Example: T0x slows down x BPM, T1x speeds up x BPM and T20 and higher sets tempo.
As this already exists (g described the effect), I'll close this request.
Excellent info! I must have known this at one time, and forgot it. Thanks for your help, g!