It's silly, but we learned this today and it may be useful for others.

To do spanish hyphenation in troff simply put the attached sphyphen.tex
at  /sys/lib/texmf/tex/generic/hyphen/sphyphen.tex

and then bind it to (i.e. use it instead of)
/sys/lib/texmf/tex/generic/hyphen/hyphen.tex before running [tn]roff.

hth