It works fine on the Oregon, I have used custom type files extensively. But sometimes it doesn't work to do this with Sendmap 20. For example, this kind of file will sometimes make my Nuvi 3790 freeze and fail to complete the boot process. I have an Oregon 400t, and have not seen this problem, but since the 450 is newer it may share the odd behavior of the 3790. I suggest that you use any test maps on a card instead of internal memory, because if this happens with a file in internal memory you could have a big problem but with a card you can just remove it.
If I create a mapset using the "correct" method with mapsettoolkit, it always seems to work. That should probably be your next stop since it will also display your map in Mapsource with the custom types.
http://sites.google.com/site/cypherman1/It may take a little while to understand how this works initially, but here's a quick overview. Create a folder and put the .img file(s) and .typ file inside. Choose that folder using the Select IMG button. Click select all and Add so the files are listed on the upper right under Selected IMG files.
Choose this same folder in the Mapset Directory box. Give the mapset any name you like. Make up a number for Family ID an leave Product Code alone (defaults to 1). Version and copyright can be whatever you like. Draw priority was discussed above.
In the TYP file box, choose the file you created with the online editor. It is already compiled so don't click that button. Check the boxes that say Install in Mapsource and Blank Overview Maps. Now click Start and if you did everything correctly the mapset will be installed into Mapsource. You computer may give you a warning about the program trying to modify the registry, so you will need to approve that. Note that when the installation is successful you get the confusing message "Convert termined" (the program author is not a native English speaker

Now you can use mapsource to view the mapset and the results of your .typ file. If you like it, send it to your GPS as you normally would in Mapsource. If you make further changes to the mapset you will need to uninstall/reinstall.
Doing this will eliminate any of the weirdness related to Sendmap20. Others have also noted these issues here.