>d) be well retained once inside cells.

This part is tricky.  A DNA stain such as Hoechst 33324 may diffuse in the
oocyte cytoplasm once decondesation begins, thereby staining both
pronuclei.  What about using a DNA stain along with a mitochandrial stain?
This works with mammalian sperm who have mitos running down the
flagella...now would it work on the two large mitos in urchin, with a
detectable signal?  It would identify which is the male pronucleus in the
event of dye diffusion.

Alternatively, stain for the whole tail.

Research is fun,
-Mark