date_popup doesn't support time fields without dates (i.e. #date_formats like "H:i" or "h:i")

Issue
If you create a date_popup time element (#date_format like 'H:i' or 'h:i:s', etc... i.e. no date field), the form value gets NULL'd when the form is submitted.

Cause
date_popup_validate() & date_popup_input_value() don't support date_popup form elements that contain just the time. They both require the date part. (See lines #311, #323, #368)

I could work on a patch for this, but I wanted to check if this was by design, a planned feature, or whether someone's already tackling it.

My use case
I have an event form & need the following from the user:
* Event Date
* Start time
* End time

I was trying to do this using two date_popup fields, with #format_date set to 'Y-m-d H:i' and 'H:i' respectively.

An alternative approach would be to set both #format_dates set to 'Y-m-d H:i', hide the date textbox for the 2nd field in the theme function, and extract just the time value on form submit, but that seems a bit of a hack.

What are your thoughts?

Would a patch to enable support for time_only date_popup fields be useful? Is there a different approach that I should explore? I'd be more than happy to work on a patch if you think that's the best way to go.