seems to me that the modifications to a person required to allow them to survive in a vacuum without a space suit are quite modest.
1. a subdermal mesh of clam like muscle cells that contract and "lock" providing constant tension requiring no extra energy, along the lines of non extension acting as mechanical counter-pressure of 0.07 bars (above the Armstrong limit)
2. sphincter organ in throat to close at 0.07 bars.
3. nictating membranes protecting the eyes
there you go, now you have someone who could survive indefinitely in a vacuum provided they held their breath.