For example, if we created a persistent multiplayer universe for Sins in which new ships, new techs, new maps, new features, new campaigns, multiplayer tournaments, new scenarios, etc. were released on a regular basis, do you think users would subscribe to this?
To me, this would definitely be worth subscribing for.
I think in general, the greatest obstacle you guys will have to overcome with a subscription model is that there are a lot of people who are against it on principle alone ("I paid for it once, and shouldn't have to pay to keep playing"). The other thing is that subscriptions are not always easy to handle when the customers don't have credit cards, and the ease of going to a store or using paypal to just buy it once will be lost with a subscription model.
Hellgate: London was the first major game to include an optional subscription system where the subscribers get extra stuff, but that caused a lot of grief from people complaining about what's fair for the subscribers to get that the regular players would not. Splitting the two would be a rather big challenge.
By contrast, Guild Wars operates on the "multiple expansions" model, and though they are bigger and pricier than what you have in mind, it works well for them - each expansion introduces a new campaign, usually new classes, skills, and most importantly features.
In truth, I believe it will be easier for Stardock and Ironclad to adopt a mini-expansion pack model, as I think it causes fewer problems overall. If it's subscription-based, you'll have to figure out what to do for subscribers and what to do for non-subscribers, and you can be very sure the non-subscribers are going to be *very* vocal about stuff they aren't getting. Doing it the mini-expansion pack way will allow players to pick up the packs that have the features and additions they like with a one time purchase. Granted, this theory largely depends on what exactly you mean by a "persistent multiplayer universe" - if it allows people who don't have an expansion to play against people with, sans the ships/techs/etc, then I would say it could be quite successful.
For me, personally, either method would do just fine - I have nothing against a subscription in principle.