i know the reg in part 61. i don't know if its different from part 141 but here it is. its 61.65(d)

"(2) A total of 40 hours of actual or simulated instrument time on the areas of operation of this section to include-
(i) At least 15 hours of instrument flight training from an authorized instructor in the aircraft category for which the instrument rating is sought;
(ii) at least 3 hours of instrument training that is appropriate to the instrument rating sought from an authorized instructor in preperation for the practical test within 60 days preceding the date of the test."

so what i get from this is that only 15 hours has to be from a CFII. I don't think that is a good idea being that the CFI might not be as good as teaching instruments as the CFII but it can be done. i know of a flight school that dosen't have alot of CFII's and the students stay with their original CFI's untill they are almost done and the CFII gets the last 15 hours and gets them ready. i think its pretty shady but i think it can be done legally.