There is a wild tshuvah in the Mishneh Halachos (vol 8) regarding someone who woke up very late on Erev Pesach she’chal b’Shabbos and was faced with the choice of washing over chameitz to fulfill the mitzvah of eating a seudah on pas before the time to eat chameitz expired or davening. On the one hand, there is an issur of eating before tefilah, but on the other hand, there is a mitzvah to wash for seudas Shabbos, and that mitzvah is potentially a mitzvah overes because there is not much else you can wash on the rest of the day. R’ Menashe Klein sets out a whole list of things wrong with the assumptions behind the question which you can mull over.
While on the topic of what to eat on Erev Pesach which falls on Shabbos, I discovered a few sources which discuss an idea which may get points for creativity but did not make too many poskim happy. Rabeinu Tam writes that since egg matzah cannot be used for the mitzvah of matzah, it is not off-limits on Erev Pesach. Using similar logic, a contemporary Rabbi proposed that matzah baked specifically “shelo lishma” which is not usable for mitzvas matzah would be permissible to eat on Erev Pesach as well. Aside from the fact that no previous poskim ever entertained this idea, the Rambam writes (Hil Ch”M ch. 6) that the prohibition of eating matzah on Erev Pesach is designed to make eating matzah on the seder night something distinctive (heker). Egg matzah is easily distinguishable from matzah shemurah. The same cannot be said for matzah that looks identical to matzah shemurah but was baked shelo lishma.