I take it you mean Laufen close coupled. floor mounted one or wall mounted?
If it's fixed to the pan by two bolts, one each side and metal plate with doughnut sized seal. NO is the answer as you tighten the bolts to pull the cistern down on to the pan it will put pressure on the repair and pull all apart.
I take it your plumber has denied responsability? if so he lost your recomendation. If you have house insurance then check out if that accidental damage is covered.
If it's not close coupled type and connected by flush tube you may get away with the repair you suggest.
Wait abit someone out in the wide world may know something I don't. means a bucket for a while but I had to for 1 month.
Bob
January 2009