To make it generic would be a matter of using the booleans to index rather than using them to multiply (as you currently are). However, for non numerical lists you'd need to reset values to nulls rather than to zero. These nulls would have to be the correct null corresponding to the type of the input (i.e. 0Ni for ints, 0Nj for longs, ` for symbols, " " for chars etc) - this can be achieved with 'first 0#'
So the generic version of your function would be something like:
q){seed cut @[(x*seed)?y;where 0<=z-(x*seed)?1f;:;first 0#y]}[3;10;0.25]
9 3 9 4 9 9 8 7 6 1 3 4 9 3 9
2 6 5 3 5 6 0 8 9 0 8 9 1 5 7 4 3 2
3 7 6 9 8 2 2 8 9 8 2 5 1 2 1 3
q){seed cut @[(x*seed)?y;where 0<=z-(x*seed)?1f;:;first 0#y]}[3;`8;0.25]
eklinmcm ikfknpam pjncfmob mmhpkfap bhakgffh khkag..
neohaicn fiajkigo bllnecdn hpnommjb pkhpildh lacif..
feinbhmg mbpkjapc dgippbmi lelmkfoe ..
q){seed cut @[(x*seed)?y;where 0<=z-(x*seed)?1f;:;first 0#y]}[3;" ";0.25]
"rk an i nts d cxfkp "
"oiusc udpliqbqnzapql"
"qhk yauhroflprr lwuw"
Having said that, this may not be the best way to approach the initial problem - this is only a suggestion for making it generic.
PS. If you still want zeros in the numerical lists then use zero-fill
q)0^{seed cut @[(x*seed)?y;where 0<=z-(x*seed)?1f;:;first 0#y]}[3;10;0.25]
7 0 5 1 1 0 3 2 0 0 0 0 0 3 0 9 7 9 7 7
6 9 4 0 4 4 0 6 2 9 5 1 0 0 8 9 6 3 4 0
5 0 8 1 1 6 4 9 0 1 9 5 8 9 0 3 9 0 0 1
