I'd suggest the -filelist myfiles.txt format to minimize the problem with yet another special syntax.

I suggested the - to keep it similar to 3dUnDump. This example is cut'n'pasted from the 3dUndump help

3dROIstats -mask mmm+orig -1Dformat func+orig'[1]' | \
| 1dcat stdin: | 3dUndump -prefix uuu -datum float -ROImask mmm+orig -

BTW, I've just noticed that that example is wrong as there are two consecutive pipes without an intervening command (| \ |), the other thing that the example above demonstrates is that 3dUndump takes a - to mean standard input whereas 1dcat takes stdin: to refer to the same thing. My preference would be -filelist myfiles.txt or - or both but not the special syntax you're proposing to keep it as consistent as possible with other AFNI programs and unix in general.