I've run temporary test tracks straight on grass several times - it's also shown a lot in LGB's older product literature - and isn't there a load of videos up on youtube from a European group who set out amazing temporary layouts on lawns?
The biggest problem is uneven lawn, of course - if you have a garden that is really flat then you're very lucky. Then there is the fact that you're going to have to mow very regularly to keep the grass short enough under the track, and to do that you're going to be taking it up and relaying it a lot (the track, not the turf....).
Other than that, it's worked fine for me! Given how fastidious we are told we should be about good tracklaying, it never ceases to amaze me how well LGB stock copes with almost any old bit of rail thrown down on a bumpy lawn!
Jon.