I was looking at some Canadian radios recently and I noticed that, in the 1930s, it was common practice, particularly by Rogers, to fit C batteries, even in mains powered radios with indirectly heated valves.

Strange! They had special long-shelf-life batteries for the purpose. Air cells??

In restorations, lithium coin cells are commonly used.

Anyone seen C batteries in mains radios here?

I've never once seen a cell or battery inside any radio that runs on the mains. I've largely confined my collecting to Australian receivers and that's probably why.

I guess if their lifetime was roughly equal to that of wet electros it might have been a viable option.

Still, it was surprising....

I think the reason might have been that it allowed proper grid biasing without needing to lose several volts across a back-bias resistor.
That wouldn't matter so much if the radio had a mains transformer, but if it was an AC/DC type, it could be required to operate with a DC mains voltage of under 100 Volts, and after passing through the rectifier and speaker field winding, there would be likely less than 90 Volts left to play with.
Some early car radio radios also used bias batteries.