Gunite Spa Design
Closing the Comfort Gap in Spas - Aqua Magazine gives a good overview of what is possible.
Spa Seat Design
Its not uncommon to have angled back rests, foot supports and rolled bond beams. There really is no limit to what is possible.
Attached spas are 100% customizable. A spa was built for a very picky lady. She put on a tyvek suit and lay in the wet plaster in the spa so it was contoured to her body.
The bottom line is you can get whatever you ask and are willing to pay for.
Paramount JetPaks are essentially removable jetted seat backs that are engineered to deliver what you want; therapy options and comfort for concrete spas.
Spa Air Bubbles
Some spas get air bubbles into the jets through a venturi system pulling air through an open pipe. Other spas use a powered Spa Air Blower for bubbles through the jets.