I have and yes I did [send the forms in periodically], if I remember correctly it was once every three months and once a year I had to pay the excess again.
I made ongoing claims for my cat for both treatment and long term medication. I signed the claim forms so that payment could be made direct to the Vets surgery. The vet asked periodically for a new form. Liverpool Victoria who I used were very good and quick on paying bills - no questions asked.
I haven't started the claim process yet and was unsure about how often to send them in. I didn't realise that you have to pay the excess every year but I didn't think about it really.
LV only charged me one excess throughout my claim. I had a renewal between starting and finishing my claim but was never asked for another excess. I think it depends on your policy.
Yes they did backdate it as when I first visited vet earlier this year, I didn't know then that I would need to claim but it turned out he needed xrays and then more specialized treatment.
I paid a yearly excess but there was no cap on the claims, either annually or for the life of the pet. I claimed every six months. Hope your little one is ok.