if you can do it with a normal map on the final asset usage, you can use an alpha image of the text in sculpt mode on a high poly to 'stamp' it into the sign and bake it out to the low poly.
otherwise you might start with the text->mesh first, then build the sign around it. For smooth text modeled in though I don't think you can avoid having a lot of verts.
If the text was popping out you could leave the text portion floating loose... actually if its sunken in, you could also leave it floating beneath the quad and do some clever texturing. on the texture, leave the part directly over the text following its outline as alpha so you can see through those parts to the text underneath, you know?