I asked if I could just fold the whisked egg white into lemon curd to make a mousse and the answer is yes, it works, but the result is - er - just like light, fluffy lemon curd. Rather sweet! I added some lemon juice which cut into the sweetness a bit, but it doesn't quite have the citrus tang I would like. A sharper base would work better.
Basically it is two egg whites, two good tbsp lemon curd and some lemon juice and it made two portions (so that's one for lunch and one for dinner!). Sorry about the naff photo - the mousse is fluffy all the way down.
More pondering - I wonder what it would be like with marmalade . . .