time interval specification in the form

Actually yes you can - if you add a input named schedule_type to the form and give it one of the following values:

0 - Realtime
1 - 2 hourly digest
2 - 4 hourly digest
3 - 6 hourly digest
4 - 8 hourly digest
5 - 12 hourly digest
6 - Daily digest

e.g.
<input type="hidden" name="schedule_type" value="1" />

Or you could present them as a <select> list.

I'll get this written up properly and added to the Help page.

I'm using a button to let people subscribe to my feeds.  Can I specify the standard time interval in the link to the subscription form (like I can do with my feed-URL)?  I need to make '6 - daily digest' the standard choice instead of '0 - realtime'.  Or is there a workaround for this?

There's no support for specifying schedules via URL at the moment, but I'll add this as a feature request.

The workaround for now would be to use a form with the schedule_type option.