Well, the whole idea is to avoid string commands, as they take 250 to 800 ns each... So definately I want to avoic turning a peek into a chr$. Might as well just deal with a string in such a case.
If performance is that critical, maybe you should write this section of code in C, and interface it with Blitz3D as a userlib. Because I don't think Blitz3D is capable of what you're trying to achieve.