Those free and useful applications could be put into the Ovi Store by their enthusiast developers. It offers the same level of signing what was available via Open Signed Online in the past.
For the donate part, developers should keep their pages, because support remains on their site, and because of the chaos in the store itself. Just instead of offering an unsigned .sis file on their page and a lengthy description of the signing process, they could simply link the download page in Ovi Store.
You will have a hard time explaining that users really find (found) it easier to sign applications for themselves (typing IMEI, captcha stuff, confirmation e-mail, download e-mail), than installing the same application with a single click.