The only possibility I can see is if the floor is left open at the end of the speeches should anyone else which to add a word. If this was so, and you decided to avail of it, I'd keep it to only 2 or 3 sentences.... literally.... very very proud and wish you both well.
On the other hand - if bride doesn't want an official Mother of the Groom (mini) speech she may not approve of an open-floor-usage.
Whatever you do, the Bride must not be offended/upset in any way on her wedding day, it would never be forgiven or forgotten.
If I were you, I'd forget about any speech. I agree that the idea sounds nice from a Mum point of view, (it could either be great or otherwise from other folk's point of view) and if the Bride doesn't go for it, enough said.
Fruit flies - help needed please.
Army horses loose on London streets
Have any of you got all electric cars? Pros and cons please.