[c-nsp] OT: Proxy server that can insert content

[Dave Temkin]

Kind of OT, but you guys are some of the best resources out there.
Does anyone know of a proxy server that I can use to insert content every
X number of say page loads or after X time period?  Looking for something
for a non-profit project to insert announcements/etc. for free WiFi being
provided to a park area, no evil intentions here...

Thanks,
-Dave