Indeed, BricsCAD actually allows DXF group codes >= 999 to be used for XRecords;
this is not compatible with AutoCAD AutoLISP, so we actually fixed that.
Nevertheless, once such DXF items with code >= 999 are present in database, AutoCAD perfectly supports them (no error in audit/recover); can be saved + reloaded as dwg + dxf files, no problems at all;
even more, ARX allows such codes for XRecords, even when the docs states differently about this (terribly outdated).
In conclusion, it seems safest to stick with DXF codes as documented for XRecords (below 500).
I hope this explains a bit more ...
many greetings !