If anyone's interested, I was finally able to get the hiscore save plugin (Recalbox 8.0.1 PC / Mame 0.235) working this way:
On your Recalbox system:
- Start any Mame game and use Hotkey + B for retroarch menu.
- Go to Quick menu / options and switch the following options to 'ON':
- 'Read configuration'
- 'Boot to OSD'
- 'Mame ini Paths'
- Go back from options and then to 'Resume' and exit the game.
- Start any game and the Mame OSD comes up.
- Go to 'Configure options / Save configuration'.
- Use Hotkey + B for retroarch menu.
- Go to Quick menu / options and switch the following option to 'OFF':
- Go back from options and then to 'resume' and exit the Mame OSD.
Now, there is a new folder in Recalbox 'share/system' called '.mame' with three ini files (mame.ini, plugin.ini, ui.ini)
On your PC:
9. Download and unpack 'mame0235b_64bit.exe' from www.mamedev.org (under Downloads/Previous Releases).
10. Copy the folder 'plugins' from the downloaded Mame release to 'share/system/.mame' (in 'share/system/.mame/plugins' only the file 'boot.lua' and the 'hiscore' folder are needed, so you can delete everything else)
11. Edit 'share/system/.mame/mame.ini' and change the value 'pluginspath' from 'system/mame/plugins' to '"/recalbox/share/system/.mame/plugins"' (with quotation marks) and change the value 'plugins' (under 'Scripting Options') to '1' (if not already set).
12. Edit 'share/system/.mame/plugin.ini' (which is empty) and insert the text 'hiscore 1' in line 1.
13. Edit 'share/system/.mame/plugins/hiscore/init.lua' and change line 25 from 'local hiscore_path = "hi";' to 'local hiscore_path = "/recalbox/share/saves/mame/mame/hi";'
Done! Your highscores are now saved in (and loaded from) the folder 'share/saves/mame/mame/hi'!
(I still wonder why noone else was facing this problem...)