Hi,
Looks like a Mono bug to me, though it could be fixed with a workaround to the bug in Mono, I think it is still a bug in Mono. Application.Lock should work the same on Mono as on .NET on Windows.
Another workaround is simply to disable NeatUpload by commenting out all references to it in Web.config. We do this for Medium Trust hosting because NeatUpload does not work in medium trust.
Best,
Joe