You will certainly need to remove star dreamer time accelleration if you want online play. If everyone is zipping around in their own time frame you'll never be able to sync up to a server.
So chances are, if its online you wont have a 'stardreamer' in E4... Unless you rethought from the ground up somehow how you would handle multiplayer, something which includes 4 dimensional player handling. but as any online player knows, it can be hard enough for a stable experience to work in just 3 dimensions.
and people complain enough about newtonian phys (if E4 will even have this), let alone let them wrestle with relativity.
(And i realise we're talking about player induced time dilation via stardreamer, rather than anything Einsteinian)
The obvious solution if you dont want to stare into space for weeks on end or make your multiplayer mode completely baffling. is to have a really accurate FTL -
and btw, anything going faster than light must be sidestepping relativity somehow so it completely gets around that issue. Or at least we'll assume that it has, so forget about time dilation, or mass increase or any of that.
IF you're close enough so your destination is no more than 30 mins real-time flight away then you don't have to worry about any of this. You could still have the option of real time flight, but i would imagine that you'd jump within-system.
If you didn't want to jump, then well we have another scifi concept that could save us... perhaps a warp drive like in star trek would be advantageous for interplanetary travel (which afaik, is not a *proper* ftl.. hmm. have to ask a proper trekkie about that) you would feel like you're flying in stardreamer, but within a warp bubble that protects you from timedialation.. so you're just going really, really fast in the same time frame - and to an outside observer, you would zip off into the distance like the enterprise as you engaged the warp engines.
So i guess thats really the best way, if you want a MMO to feel like frontier... jump between systems and warp when in system until you're close enough to fly in real time on sublight engines. That way it takes the same amount of effort as before, plays and feels the same - the warp controls could be exactly the same as stardreamer for example and require use of thrusters to push the warp bubble / ship. but everything is kept in a nice neat consistent time frame. so essentially it could be exactly like stardreamer, but without time accelleration.