right lets start,
people have modded the antec 300: have a look at this
http://www.themodbrothers.com/viewtopic.php?f=14&t=15
personally no i have not cut holes in the mobo tray though i have seen pics of a build where someone has. but to be honest you probably wont need too, just get a modular psu. (will try and track down the pics).
i think someone on here was going to cut a hole below the psu, but to be honest its just going to pick up loads of dust. also if you have to have the psu the right way around there is a few mm of space between the psu fan and the case.
why would you want to cut grills out? you can take the fan cage out but i think in the modding blog (link above) he was doing something like that.