Ya, the only downside however it could always be put in a small popup sort-of off screen, or heh.. loaded on the same page via AJAX.
Download Link > Loads AJAX based "wrapper" > Displays Banner > Click Banner > Banner Page opens in AJAX "wrapper" > Banner is replaced with the download link and the Banner Page remainds (below it) in the AJAX "wrapper"
Id say my concern isnt if the 'ad people' find this to be a tos violation, maybe they wouldnt as the banner's page is opend on the same page the user is, but thats the way I'd do it
This would resolve popup issues (blockers and the fact it disrupts your process).
Sounds easy