Dear Rabbonim shlita
The Shulchan Aruch brings down that where there is a minhag, no meat or chicken should be roasted for Leil Seder. The Shaarei Teshuva brings that if at the beginning of cooking, the food is covered in liquid, that’s ok. What exactly is permitted? What if you only pour a sauce over the food? What if one is eating by someone else who doesn’t know the halocha, can one eat if they made a dry roast?
The custom, as you note, is not to eat roasted food on the Seder night, but only if the food was dry at the outset. If the food was covered with liquid at the outset, there is no problem.
Furthermore, poskim debate whether the custom applies even to roasting in the oven, which does not qualify for the Korban Pesach. The Mishnah Berurah (476:1) is stringent, but the Aruch Ha-Shulchan (476:2) is lenient.
If the food was roasted without sauce, it will not help to add sauce later. However, as noted, water or juice at the outset will alleviate the problem.
When eating with somebody who does know of this custom, one can be lenient.