May be you've seen this YouTube video:(Author is not exactly correct on when overbright feature was cut from Goldsource though. Early 00s Steam builds still had it, I was introduced to HL with Condition Zero when it was already on Steam and still had overbright.)
Posting this video on HLFX forum started a discussion that resulted in fixing display of lightmaps and in couple other bugfixes like lighting of outdoor models.
Thanks to user Decay for lurking VOGONS forums and figuring out how to implement it in Xash3D.
More screenshots: https://imgur.com/a/ahntAbu