John, it would be very easy for you to do this yourself.
- Re-name register.php to your desired file name.
- Search and replace vbulletin-style.xml for register.php to your new filename.
- Search and replace vbulletin-language.xml for the same file.
- Re-upluod style and language via ACP.
- Search and replace existing template customisations for the change incase you have custom edited a template which contains the filename, which would not be affected by the re-import of the style xml file.
Each time you upgrade you will need to perform the actions on the style and language files and re-name your register.php before uploading. It may not even need the change to the language file at any point; I don't know off hand if there are any references to register.php in there.
I have renamed quite a few of the vB file names, it seems to give me improved Google results too - maybe because it spies something a little different and not the out of box solution.
Although personally I would like to see it become a part of vB settings, where you can name each of the files individually and a variable would be used in every file name, template and phrase to identify each filename.