Although he's used a number of basses throughout his live career, Gene has managed to sneak in a few more that have appeared only in the promo videos. These are presented here in (roughly) chronological order.
Gene has been a Spector fiend since 1977. For the most part he's played the three custom SB-1's that was built for him but for some studio work and the occasional video shoot he has donned a Spector NS-2 or two. In 1986 he brought a red NS-2 to the video shoot for Tears are falling. Considering the costumes and the studio "landscape" the bass may be the most understated thing in the video.

A similar NS-2 but with a white finish and two J-bass pickups was used for the promo video for Who wants to be lonely in 1985 which probably counts as indirect evidence that Gene aquired both basses at this time. When it was time for a more subdued feel for the Forever promo in 1990, Gene sat down on the stool and wielded the same white NS-2.
Curiously enough there is scant photographic evidence of these basses in Gene's hands except for stills from the above video shoots. The only other pictures I've found are these from the photo shoot for the Asylum tour book. No live pictures or video exist of Gene with the white NS that I know of but what appears to be an NS-2 shows up standing off-stage in at least one Asylum bootleg show (see the 1985 clip of the Jackson Axe elsewhere on this site for a brief glimpse of it). A red NS-2 of similar specs, but different from the one in the previous video, seems to have found more favor with Gene even getting use on stage on at least one occasion. Considering that these photos show the 2nd Asylum stage it was used between December 11, 1985 and February 2, 1986.
This bass has a long and checkered past considering that it appears so seldom. I call it a Valdez based on the headstock design being the same as on the Valdez Axe (which is conjectural but still). It makes its first appearance as a prop in an ad for Sunn amps in 1979. After that it shows up briefly in promo shots in 1983 and 1989 and disappears again after being featured in the Hide Your Heart video. It was auctioned off at Butterfields with the description: "An original 'bat shaped' electric bass guitar with a silver pearl 'skull and crossbones' inlay on the top face of guitar."
To be perfectly honest I have no idea what this bass is. Neither the headstock nor the body are clearly visible in the video and calling it an ESP (entirely based on the overall shape of the logo on the headstock) is just a shot in the dark. Not even this posed shot from the video shoot shows the headstock well enough. My guess is that it's just a loaner that was "used" for the video and then returned. The fact that Bruce plays a bastardized version of a George Lynch model ESP seems to support that notion. If anybody can identify the bass, please let me know.
For the recording of the video for God Gave Rock n' Roll to You II Gene pulled out another loaner, this one borrowed from Silent Rage bassist Ej Curse. (There are plenty of pics showing Ej with the bass and even a pic with Gene and Ej with the bass.) It looks like a relatively early B.C. Rich bass judging by the 2-a-side headstock and the 'R' logo. The pickup combination, clearly seen in the photo, suggests it's been modified . (I'm no B.C. Rich guru but my guess is that this Warlock is pre-1986.)
The red NS-2 evidently got the standard "live" treatment with an added bridge cover.