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Yes, you'd have to cut the lines from the open-collector outputs to the LEDs, but I think the same logic principles will work. For example, if you place a PNP Q as a series switch between the 8T06 output and the "A" segment (top) LED, with the emitter toward the LED, and the base driven through some resistance by BCD 2, it should disconnect the normal decoding for 6 and allow the top LED to go on in the desired states.

This could get tricky though, since there's not much operating voltage available. The LED may be at about 1-1.2V on, and you'd need B-E voltage for the PNP, plus base resistor drop, plus output voltage drop on the 8T06. There's not enough to run a Darlington, but if a P channel MOSFET with very low Vt could be found, it might be doable directly. Or, maybe the PNP could be arranged as an emitter follower from the 8T06 output to the LED, with collector grounded. With some base resistance from the 8T06, and a diode from the logic input, it should override the normal state.

Certainly, a slightly more complex circuit say with an additional transistor for each for gain, level shifting, and polarity control should work. Or, some kind of CMOS switch with low enough Ron, or a MOSFET power switching IC may have what it takes without any additional parts.

Of course, it all depends on the situation. If this is a one-off circuit mod, then no big deal, but if there's a whole bunch of them it's another story.

Ed