/ialfill has no other parameters or options. At this point, I would rather make sure it is working correctly in its simplest form across all ircds and may consider adding extensions later on.
Good point about the T parameter. I'll set it to a fixed number.
Yes, WHOX can return different combinations of parameters and there is no way for mIRC to know what they are. mIRC performs multiple checks on the WHOX 354 reply to verify that it is in a format it expects before parsing it as an /ialfill request. As you say, it was hiding the results for any T request - this will be fixed in the next beta.
