Of course you can. We have stayed offsite several times and haven't had a bit of a problem making early PS's for the MK. In fact, I really like making an early breakfast PS for MK on a nonearly entry day. It's great to get in the park early.
If you are parking at the TTS, they will probably ask you if you have an early PS before you can hop on the monorail. And they will probably have a list of the PS numbers at the gate before you enter the park (usually its the last turnstyle to the left). 'So make sure you bring your PS number with you.
Have fun!!