Specify protocol (webcal/https/...) with formatted_abc_url()?

Hello there,

is there a way to specifiy which protocol a
formatted_user_events_url(@user) uses? I am asking because I have a
custom format.ics {} declared in a controller and would like to
specify a different (from the standard http://) protocol…
specifically webcal:// for that one. Is that possible?

Cheers and thanks,