If Massport were to put customs in terminal A, we would probably see a lot more. Sky Team controls a large share of the Boston-Europe market. But until A gets the facilities, the LHR and AMS flights are all we will see.
But, if they were to get the facilities, I could see DL setting up a decent sized mini hub using 752As and 763s. Brussels, Manchester, Dublin, Milan and Lisbon on 752As and LHR on 763s could work. Brussels is one of the largest European cities without non-stop service to Boston. AA has even taken advantage of this with tag on service from LHR. So a person can go same plane BOS-LHR-BRU.
That was DL's plan way back in the day. They had planned, when announcing the construction of the new Terminal A, a gradual ramp up of service. Of course, that was before jetBlue and long before Southwest came along and trashed the yields of the big boys at Logan. So, this is certainly interesting news, but unless we start seeing a commensurate number of domestic feeder flights, I'm skeptical that anything more will come of this on the international front, except with the possibility of an NRT flight, should no one start that route up in the mean time (ANA being a strong contender, being the launch customer of the 787 and all).
As for FIS in A: It ain't gonna happen. It's not just Massport who was reluctant about it after dumping a ton of money into the Terminal E expansion, but from what was told to me directly by CBP guys a few years ago when I worked at Terminal E, they're not too keen to start running a split operation. So, even if Delta and Massport built FIS facilities in A, there's no guarantee the CBP would staff it.
That said, I'm sure that's a relatively small hurdle to jump. The bigger problem that remains is to ensure enough traffic to make such an investment feasible, you'd need more than just the LHR and AMS flights. You'd need to get Air France and Alitalia over to A as well. Easy enough, but where are you going to put DL's 767s and A330s, AF's 747s and (eventually) 77Ws and AZ's 767s and A330s?
Unless DL pulls down its mainline to the point that they could ensure that the domestic operation could be run from one side or the other, there's no way you can get the traffic necessary to make FIS investment worthwhile, simply because the reduction of gates in order to accommodate the widebodies would cripple the rest of the operation. Unfortunately, once the FIS ship sailed, the gates were set up with a primarily narrowbody operation in mind and that will be a huge barrier to any increase in international traffic out of Terminal A.
That said, the airliners.net thread, to my knowledge, didn't show the press release with a preliminary schedule (obviously, exact times will be based on the slots negotiated with AA/BA). I would have expected either something a bit earlier, around 18:30 and 20:45, or keep the 19:30 departure but pair it up with something around 22:30, to capture the business travellers who want to be in the city all day, seeing as DL probably won't be expecting too many connecting passengers in London - that said, 21:50 does make it the last departure to LHR, based on BA's traditional schedule.
Anyway, it'll be interesting to see what happens. With AA most likely exiting the market and BA filling the gap with an additional flight, there is definitely some wiggle room in the market, one of the largest transatlantic markets out there. Delta, furthermore, has a pretty big frequent flyer base to tap into, though their main handicap will be the fact that they cut back their domestic operations the past few years, meaning they don't have much in the way of connections on the route beyond what they can route in from elsewhere in the US and what their SkyTeam partners can do on the LHR side of things (SkyTeam are the smallest of the three alliances at LHR). Of the airlines serving the market, AA/BA are the strongest, tapping into AA's feeble connections at BOS but BA's massive network at LHR and Virgin Atlantic with some longhaul connections out of LHR. Therefore, DL of the three of them, will have to try the hardest seeing as this is primarily a point-to-point (P2P) route for them. That said, I'm cautiously optimistic for this route's success, though I wouldn't be surprised if one of them downgauged to a 757, especially during the winter.
Anyway, that's my analysis. Sorry to be so long-winded.