You certainly have access to most of the sensors you would need to do an INS for the glider. A pressure sensor for measuring the depth of the dive would probably be useful. If your hull compresses during the dive you might be able to do it via measuring the strain on the inside of the hull rather than have a penetration through the hull - minimise these, get a leak and you can probably kiss your glider goodbye. BTW, you might want to keep an eye on your glider's seals they apparently only last a few deployments.
I imagine currents pushing on the glider during the dive would be something you might have to live with and compensate for on the next dive after a GPS fix.
The ability to roll the glider during the dive might help as well - maybe by moving a weight around the long axis of the glider?
Have you done a search for papers on how other gliders like iRobot's Seaglider, etc navigate? Does your university have gliders already? If so, are they going to let you pull it apart... I mean examine it?
Of course, its all a trade off - the more electronics you put in the more current you pull from your battery and the shorter the deployment of your glider. Hope some of this helps.