Are you using 3.1 or 3.9? If 3.9, I think it is related to Workbench. Icon.library does not care about icon type, really.
I just did a test - it works perfectly as I want in OS 3.1.
* I booted from Workbench3.1 in DF0: and have Extras3.1 in DF1:
* I copy WBStartup from Workbench3.1 to RAM Disk by pulling it over
* I rename WBStartup in RAM Disk to "Project"
* I open IconEdit from Extras3.1:Tools, I open RAM:Project.info
* I go to "Type" menu and set it to "Project", I save the icon and exit IconEdit
* I double click the Project icon in RAM Disk
- Screen flashes and the screen bar says "The icon(s) have no default tool"
* I click "Project", and open Information, sets "Default Tool:" to "Multiview".
* Again I double click the Project icon in RAM Disk
- Multiview opens and shows content of "RAM Disk:Project" - as I expected.
At some point on the way from OS3.1 to 3.9 this behavior vanished.
EDIT: Correction - it works by doing a work-around suggested by Peter K - rename the drawer from shell, set type of icon (that now lacks a corresponding drawer) to project, give it a default tool, rename the drawer back in the shell. Presto, it works!