@Stuart,
Do you mean you want the Page Selector Form Control to return the path instead of the Guid?
That control doesn't work that way, so if you wanted that behavior you'd have to create your own custom Form Control. You could try copying the existing one and start from there.
If you did have it insert something like the NodeAliasPath of the selected page, what would happen if the page was moved or renamed? In these cases, having the Guid value is safer since you can always look up the 'current NodeAliasPath' from the Guid.
Side note: You can change the value of the custom Page Type field to Unique identifier and still use the Page Selector Form Control so that you don't have to convert the string value to Guid in your code.