It works now, sorry about the confusion. In the client, I am using a "dummy" user. In IPFS, my corporate user was able to submit the file fine, with the custom name. It's wierd, I can do the SubmitToSharePointList command with my dummy user.
Please understand that the anonymous access problem is not related to this at all. In IPFS, the SubmitToSharePoint list with ANY users when anonymous access is activated.
Anyway, the filename feature is great. Thanks for all of your help!