There's no line labeled "luminance" on the VGA card's pinout, but by definition whatever color line is connected to the monochrome monitor thus becomes "the luminance line", since luminance is all the Mono monitor understands. (And on a VGA monitor's pinout that line will probably be labeled "luminance".)
I always forget which of the three lines is the one the mono monitor connects to... it's been twenty years, cut me some slack. :^b One of the reasons I'm skeptical about the contention that leaving the lines hanging will hurt anything is I actually made my own mono VGA cable once (it was for a grayscale NEC Multisync GS that had a nonstandard 9 pin connector on the monitor side.) and I would *swear* that when I was fooling around with it (I was young and stupid and willing to simply stuff stripped wires into holes to test if I had it right) that when I had the mono select pin grounded I could connect the luminance line from the monitor to *any* of the three color guns and get the same picture. If that memory is true it implies that at least the old Trident VGA card I was working with didn't bother downing the extra lines, it just locked all three palettes together.
The Linux on Obsolete Displays webpage shows an interesting exercise in using the three color lines as *separate* luminance signals. Scroll down to the "Poor Man's Triple Headed Display". He does it using TVs, but he notes in principle you could do it just as easily (well, more easily) with three mono VGA monitors.