The normal way of securing a toroidal transformer is through the hole in the middle of the toroid, so may not be a good place. Also the transformer generates heat which isn't the best thing in the world for an electrolytic capacitor.

Otherwise go ahead and put the capacitor there, it isn't going to matter a great deal and there is no "electronic" reason why not.

Just be careful when securing the toroidal transformer that you don't create a "shorted turn" . Easy to do if you are not aware of this potential problem. It won't do the transformer any good at all!

Andrew.

There is no field in the centre of a toroidal transformer – that's the point of them. BUT I think you could get some very odd effects if the cap is the usual metal case type with the negative case grounded to a metal bottom plate and fed by a bridge rectifier from the secondary. Essentially ANY conductor laced through the centre of a toroid couples very strongly to the windings and the cap case could in effect be a winding. So I would avoid it.

The usual mounting kit for a toroid is a couple of big insulating washers and a sort of pressed tin "hat" arrangement with a single screw through its centre to clamp the toroid down – ensure the top plate is only connected through the screw to ground to avoid a shorted turn.