It would be difficult to stay stable for any length of time because those basic instruments give you almost no information on your angle of bank. 

If you could manage to keep your spirit compass on a constant heading and keep the yarn straight, your wings would be level. 

So while it would be possible, I would think you would need an experienced glider pilot and relatively calm conditions to maintain wings level for any usable length of time. 

It would be interesting to hear from someone who has actually done it.