John, you sort of can as an initial setup but it can be difficult to manage. You are correct in that you cannot access the assignment record, but you can initially setup a resource with a % allocation. It is accomplished by setting the duration and hours of the task first before you assign the resource. Doing that will assign the resource for that duration and work, which backs into an assignment percent.
Now managing that is a bit more difficult. Assigning multiple resources and changing the duration can be challenging and the only way to “fix” a situation is to recreate the task or use Project Pro.