Can't help you there, I don't have a DT25 or DT50 and I never will have. I don't need it to get great sounds. I do like the concept of it and I'm sure it sounds great but so does the L3t and L2t. I've done experiments with my other guitar players HD500 using line 6 link which automatically put the L3t in guitar mode. If I remember correctly I think it automatically bypassed the HD500's speaker modelling. It sounded awesome but it also sounded great using the speaker modelling going into an input on the L3t, so in that respect it's very versatile.
I think once you get the L3t you'll better be able to decide if you actually need to use both, but if you are determined that you do need both I don't think the L6 link would be the way to go. Isn't there a direct speaker out on the DT series amps? That would likely be the better route to go, I have a feeling that if you did try using the L6 link, and it worked, it would be telling the L3t to go into guitar mode all the time unless you spend a bunch of time setting things up so that it doesn't. I think you would have to do something to every patch to make the L3t stay in PA mode. If you just use the direct out into an input it will just help make your DT louder, if you even need that.
Once it arrives try all the scenarios and see what's best and hopefully report back, I'd like to hear about your findings. You may just find that using the HD500's speaker sims through the L3t in PA mode kills. Maybe not, it will be interesting to see what you come up with.