A boring way to do it would be to have 12 events (one for each month) repeating on annual cycles. It'd take a moment to set them up, but it would look the way you want it to, I think.

If anyone knows a more elegant solution, I hope they speak up.

Unfortunately I don't think there's another way to do this -- repeating on the last day of each month is a special case, so we'd have to look at adding support for this option (it would be useful though).

This would indeed be useful; and even more so if there was some way of setting a due date X days before the end of the month, not necessarily just on the last day. (For example, our time sheets are due a few days before the end of the month. To complicate things, it's actually not calendar days, but business days... don't know if there's any way to address that...)

I also have been missing this feature already. Would be great if you could add it.