I emailed IFTTT support about this and here's what they said:
Really cool idea! This isn’t currently possible, but we’ll keep it in
mind. Thanks for sharing your feedback.
Your Recipe will check your feed every ≈15 minutes for new
information. I hope that clears things up. If you have any further
questions, please let me know.
I know that email-to-RSS services like MailChimp and Aweber often look at the published date on the RSS item and only retrieve it if the date is within the last 24 hours. If possible, you might want to try modifying the date on your RSS feed items to something within the last 24 hours and see if IFTTT picks up more items. (You might be able to use Yahoo! Pipes to modify the date on-the-fly and give IFTTT the Yahoo! Pipes feed.)
Otherwise, creating a PHP script that does what you need is probably the next best option.