isn't the plastic cup just a stabilising spacer to assist even clamping,Like you say the rubber o ring seals the join.As long as the o ring is the same size as the standard injectors then your picture of what you are thinking of doing should work fine in my opinion
.The clamping on the o ring is very small anyway,What cc injectors are those out of interest.
I never really looked into it but doesnt the dropping resister on the s13 have something to do with changing high to low impedance on injectors?