You can use the clear command to allow you to reuse the same memory area, e.g.
Code:
loadaddr = &6000
CLEAR loadaddr, &8000
ORG loadaddr
INCBIN "file1"
SAVE "file1", loadaddr, P%
CLEAR loadaddr, &8000
ORG loadaddr
INCBIN "file2"
SAVE "file2", loadaddr, P%
etc
I know it looks a little clumsy, but it will work.
Of course, the other option is simply to put the files on the template disc image (specified by -di), which was exactly what it was intended for in the first place. But I guess this isn't so convenient if you're changing the files' contents all the time.
Perhaps in the next release, I'll implement a PUTFILE <host_filename>, <beeb_filename> command, although it'd be unusual in function, compared to the other commands in BeebAsm (because it doesn't affect the memory image, nor the assembly).