Hi,
We recently stayed at the Husa President Park Hotel - it was near the Gare Du Nord - about 10?? minutes away from Gare du Midi. It was a bit shabby for a supposed 4 star hotel and there were renovations going on. But overall, it was an alright place to stay for the price.
I booked the rooms on lastminute.com and they were about £176 per room for a 3 day stay. It was some sort of deal that only lasted that night, though, so not sure it will still be that cheap.
I hope you have better weather for your convention than we had for our visit. It was pouring and windy - not very good for sightseeing (although, from what we did see, Brussels wasn't good that anyway).
Good luck!!