Since 7.0 Calc has "Very large spreadsheets" experimental option (under Options-Calc-Defaults).
(It's still buggy as seen in bug 133764 and regression bug 133619.)
There should be an option in command line to start headless LO with or without that jumbo option.
ON a second thought, it would be better to add option to use Experimental features or not, and than add suboptions like this one for jumbo sheet option.
For such options, the only reasonable comment-line solution could be to use environment variables. No command line arguments should be added. But environment variables are fine; and we already used those - e.g., when testing harfbuzz IIRC.
I'll take that as confirmation and retitle to environment variable for experimental support.
Further there should be possible to chanage: Calc jumbo sheet, Writer outline (those 2 are experimental as of 7.3+), Calc Multi-Threaded Calculation, OpenCL.
Per list there exists: SC_NO_THREADED_CALCULATION, SAL_DISABLE_OPENCL.
SC_NO_THREADED_CALCULATION doesn't seem to work, it cannot disable Multi-Threaded Calculation.
But while SAL_DISABLE_OPENCL=1 can disable OpenCL, it cannot be enabled.
FTR: https://opengrok.libreoffice.org/xref/core/vcl/README.vars?r=d4afd3ae lists environment variables affecting VCL. There are e.g. things to enable/force Skia or RTL UI.
https://opengrok.libreoffice.org/xref/core/include/sal/log.hxx?r=95e26d3d#206 describes the complex syntax for SAL_WARN environment variable, which controls the amount of logging to output in debug builds.
So having a set of variables (or a variable taking complex value) to control experimental features would be in line with this already existing approach.