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Are you suggesting pulling up the signal with a voltage gain?
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With an offset. Up or down, I always get confused which way is lean and which way is rich..It is always the opposite of the opposite in my head, anyway.
and I always have to look it up again.
I mention it for two reasons, one, I have read about it being done(never done it myself), two, I have seen a toyota technical service bulliten which talks about "oxygen sensor offset", where an oxygen sensor will develop a permanent shift of what voltage it gives out when the ratio is stochiometric (sp?). Now in the toyota TSB they look at this as a problem, because the mixture is not where toyota wants it, and they show how to look at the waveform with a scope to detect the issue.
I would think if you wanted to try it, a single 1.5 volt d cell battery, with a 10 or 20 turn variable resisitor (pot) of perhaps 10K ohms or 100K ohms around it as a divider (not just in series) so you could adjust the output voltage, would work. If you wanted the ability to go BOTH positive and negative, then 2 D cells in a bridge with the pot (again as a divider, not in series) would allow you to go either way, with the pot center as zero offset. D cells should last about forever in this application, but I expect they will change voltage slightly with ambient temperature/phase of moon/whatever. If you decide to try this, Whatever you do, start at very low voltage offsets and move up cautiously, you dont want to hurt anything.
Just an FYI, I have been soldering wires to the ends of D cells for a long time now to avoid the eventual but inevitable contact problems you can get at very low current flows. If you want to try it, first make a wetted puddle of solder on the d cell, then come back later and put a tinned wire in the puddle. Work quick and you won't heat the d cell much. Wear glasses etc. if you are worried about rupture, but my *opinion* is that there is not much risk if you work quick.