I just found out right now that you can sign players to 10-day contracts in Association mode when you are not over the salary cap. But the problem is after selecting the 10-day contract option, it would look like this.
Total: N/A
Type: Back (+4.5%)
Years: 10 Day
Option: N/A
No Trade: N/A
Note that the "MLE Remaining" is not seen. And even if the interest bar is full, no matter how many times I press start to offer the contract, it won't accept it.