I don't want to have to have one form on the register page for the main simple text fields, and then another for the file uploading of the CV / photo...
I was thinking of alternatives until I hit that clause.
I like two steps, prevents knucklehead spammers and makes things easier for my MODx lifestyle.
I say deal with members, not mere registrants; then you can do what you want with the update profile page.
The update profile page has unique data about the user which you can use to better define specific upload paths.
I remember using FileUpload to set username path and specific/allowed file; with registrants, I could not bother how to use temp whatnots.
Why, you could even use a heavily stripped (or not) version of elFinder to upload and maintain any file of choice.
TinymceWrapper makes it possible to use some cool file upload/browsers in the frontend, with or without TinyMCE.
If you reconsider this path I am suggesting, you will find more people here who have accomplished similar tasks.
I like easy!