I think it won't be so difficult to add option 'play tracks from interval playlists' when creating sheduled or immediate playlist.
There is actually already an experimental feature to allow this, although (as I said) it's experimental, and there is currently no UI for it so you can only administer it via the MySQL console. So it's not for the feint of heart, but if you want to try it...
First, you need your the account ID for the account you want to enable it on. Get that by running:
SELECT id FROM accounts WHERE username='_USERNAME_';
Replace _USERNAME_ with the actual username of the account.
Then, run the following queries:
DELETE FROM playlist_priorities WHERE accountid=_ACCOUNTID_;
INSERT INTO playlist_priorities (type,accountid) VALUES
('now',_ACCOUNTID_),
('scheduled',_ACCOUNTID_),
('interval',_ACCOUNTID_),
('request',_ACCOUNTID_);
Replace _ACCOUNTID_ with the account ID you looked up above.
The now/scheduled/interval/request lines represent the priority of playlists of those types. So in the above (default) order, 'now' (immediate) playlists override everything, then 'scheduled' override everything else, and finally 'interval' playlists override tracks requested via the song request form. General rotation playlists are always used last -- all other playlist types always override them.
So as an example, to do what you wanted (interval overriding scheduled), assuming your account ID was 9999, you would use:
DELETE FROM playlist_priorities WHERE accountid=9999;
INSERT INTO playlist_priorities (type,accountid) VALUES
('now',9999),
('interval',9999),
('scheduled',9999),
('request',9999);
As you can see, I've moved 'interval' up above 'scheduled'. (It's complicated to explain, but easy to do.
)
It's not received much testing which is why there is no UI yet, but the plan is to eventually make this configurable in the account settings.
HTH..