I don't think this can be done. There are not any modifier keys on a ipad besides a shift and cap lock. Also the keyboard only appears when you are actively in a field. Whenever you type something it gets entered into the field.
Any word on if FileMaker Go will get keyboard shortcuts with the external keyboards? That would be wonderful if shortcuts were added.
I recommend posting this suggestion to the Product Ideas board at: Product Ideas
The Product Ideas board is monitored by Product Management and Development. All feature requests are read, discussed and considered for possible implementation in a future release.
The onscreen keyboard very quickly becomes extremely limited. I have done quite a bit of iPad development now, and the standard keyboards are both limited in their content, and the way they affect the screen size and lock. I now tend to disable it and use 'popup' custom keyboards containing custom buttons, that do pretty much whatever you like. The best example I have is the requirement for an electrician to have a numeric keypad with 0-9 buttons, but also a LIM, N/A, and > button. That's quite specific, not something FM are going to do out of the box.