Not only would nicotine poisoning be an issue, but it would be ten times cheaper to use rue or caapi anyway! Just using the tobacco you might have lying around wouldn't do the full trick.
Although you do mention using it to potentiate MHRB. Does that mean you're thinking of making Jurema, just using pure MHRB, or were you asking about using tobacco as the MAOI in an ayahuasca analogue? Because it could be possible to use tobacco as an admixture plant in Jurema, as you could use small enough amounts to give nicotine poisoning a wide birth. However, given how low the MAOI levels are in tobacco to begin with, it may not add much more than a nicotine buzz to the experience.
HOWEVER, if you or your familiar do want to attempt such a thing, DO THE RESEARCH FIRST. Your familiar should FIND HOW NICOTINE REACTS TO DMT before attempting such a thing, and how harman reacts with other MAOIs. No offense intended in any way, but you sound like you're looking for a couple of short-cuts. My recommendation is to just take the time to do everything the right way, almost not matter what you think now it WILL be worth it in the end.