Thanks, all, for the info and tips. I jumped at the kits and found the following:
1) the kits are easily obtainable from your dealer
2) as noted above, the parts list does not specify or recommend fasteners
3) holes in the under seat frames are 5mm (SAE: 3/16), and as noted above, not threaded
4) fasteners longer than 9mm (SAE: 7/16) will jam against and interfere with operation of the adjustable bolster on front edge of Sport seats, rendering the bolsters inoperable
5) spent 8-9 hrs in various parts/auto supply houses looking for plastic fasteners to fit - unsuccessfully
6) with assistance from a great parts guy, Andy Ucci, at Towne BMW in Clarence, NY (suburb of Buffalo), found that BMW P/N# 51-16-8-220-001 works as if these fasteners were made for this particular job. They are 5mm self tappers, 9mm long, with 10mm hex head and self washers
7) installation takes 5-6 minutes using a 1/4" drive ratchet handle or straight handle driver. Piece of cake!

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Sageriders
2011 328, Water [blue], Saddle
2006 X5 4.4i, Sterling Grey/Truffle, Premium, Sport, Cold, Rear, Adj. Ride Ht.
2005 X5 3.0, Sterling Grey/Black, Sport(Totalled!)
2002 330i, Grey-Green Met/Truffle Br. [Sold]