Have offer temporarily replace base rate in Portal signups

It would be great to have the option to promote an offer to temporarily replace the default subscription cost for a tier in portal for new signups.

For example, I like to run an offer so that the new users can get their first 3 months free. I do this using the offer functionality. While I can link directly to this offer URL at various portions of the site, if I leave the portal enabled (which I prefer to do) there will be two monthly plans being offered on the site at the same time – the stock rate, and the offer for 3 months free. I’d really prefer to have the option of (in this scenario) all new monthly accounts coming in on the 3 months free offer, which I don’t know how to do unless I disable portal functionality entirely.