Yes it can be possible without touching the fundraising's banner.
The banner can be put in the Mediawiki:sitenotice in each project and can cohabitate with the fundraising's banner.
The real problem is that a small banner can have only a limited effect.
On 18.08.2015 23:31, Platonides wrote:
Given that they are going to take you banner time, maybe they would be possible to be convinced to include a small "WLM ad" in the donation page?