Actually, the timing was just about perfect for DT to open for Rush back in 2007, and I mentioned this to MP. But MP said that while they'd jump at the chance, Rush weren't interested because they enjoy doing their Evening With shows - this I might add, after MP had established contact and somewhat regular communication with Neil Peart.
Even if there was such a scenario where Rush still did a 3 hour show and DT opened with a 2 hour set (or even just 1 hour), DT would have to follow Rush's pace of not doing more than one show at a time. If you look at Rush's schedule, to help Geddy preserve his voice, they no longer do 2 shows in a row, even if they play multiple dates in the same city. And then Rush usually takes 10-14 days off for every 4 weeks (or so) that they are out. Whether DT would be willing to commit to that schedule would also have to be considered. They could fill in some of the non-Rush dates with headlining dates of their own (not unlike what AdMob was doing with Godsmack), but DT only goes out for 4-5 weeks per leg with a month break in between. So again, would they be OK with changing things up? I'm guessing the answer is probably yes.
However, as I mentioned before, the perfect opportunity already happened in 2007 with nothing coming of it. The fact that they are now label mates means nothing to Rush. That said I'd put money on it, that the reason why Trivium got the opening slot for DT was because they are on the same label as DT, but considering that MP is no longer hand picking the opening acts, the DT guys probably are happy to let their label pick someone for them (bad choice, IMO, but what are you gonna do?)