I wonder if there was a compression lifter on the valves, (like on some old big single motorbikes), so that there was no compression on the cylinder having the plug changed?

You could well be right.

Wright Aeronautical Corporation - Compression-Relief Device for Internal-Combustion Engines

Now there's a nice piece of research.

I'd still be surprised if even a compression - relief valve would allow fitting a plug.

They had all the time thay wanted to do this so my guess would be a climb to 10,000 ft with mechanic outside and ready, shut down and stop the prop, change plug(s) in the glide and restart. Repeat as necessary.

This link to the efforts of the previous year 1929 when the endurance record was broken 9 times!
Interesting to note the comments about exiting the aircraft to grease rocker arms and "adjust" magneto and spark plugs.
https://generalaviationnews.com/2015...rance-flights/

Video
https://dissolve.com/video/World-end...01-D378-15-096

A family affair
https://daysgoneby.me/have-you-ever-...from-illinois/