I rode a tarmac sl6 with draft legal clip ons until I recently got a tt bike, using a 0 offset seatpost and a fabric tri saddle. got into an ok position, passable. I'd be more worried about getting stack height right at the front etc. it worked well for the olympic I used it for! This was essentially my road position with the TT bars added, so if you are aiming for a properly dialled in position it could work quite well. The tarmac sl6 is an awesome bike!

You say 2020 Tarmac. I assume that is an SL6 ? (not the just-out new SL7 which I think is deemed 2021 ???).
Is it an S-works with the aero post ? Or a plebs one with a round post. (I have the plebs one 😉)

If the round post... then here's what I used to do before I got a TT bike (not on a Tarmac, but the same principle)...
I got a Thompson layback seat post (the post with a kink part way up the post to get the layback). Then rotated it 180 degrees to give a forward position. Then used which ever saddle I liked (a Cobb Flow Max as it happens).
I simply swapped both post and saddle as a combo when I put the clip-on aero bars on and put a band of black tape to mark the insertion depth.
I always felt safer using a well made Alu post for that given I was using it the wrong way, than trying it with a carbon post. At least if it was to fail it would be a ductile bending of the post, not a catastrophic sudden carbon fibre enema and crash. Never a problem for me though- and I'm a big enough guy (6ft, 190-200lbs at the time)

I did my couple of years of tri like that inc. My first IM, before getting the TT bike. I may do that again this year for a particularly hilly race I have.

Correct - 2020 model; but it’s the Disc Pro that has the aero (or at least not perfectly circular) seat post.

I’m thinking a zero offset may get me pretty close to where I would like to be, but wasn’t sure what other options exist.

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@CycleHeavy

A noseless saddle (Adamo, etc.) will typically get you a more forward position (~5cm) than a traditional road saddle (when both are pushed all the way forward on the rails). Depending on how much you need to move forward this could help bridge the gap.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

Glad you asked this question. I had thought about it myself. However, I complicated things more since I bought a comp version and then added the aerofly II bar. I love it and am glad I did. However, that complicates my situation as I don't think I can add any aero bars to them.

I could get the venge stem and then the venge clip ons, but don't really want to.

I bought a tri bike but haven't given up the idea of using my tarmac to race and just deal with being that guy on a roadie (that is hopefully passing guys on "fast" tri bikes!). I would like to get the SL7 and thought of the venge ,but don't feel the venge is worth the money it's going for.

Keep us posted.

Specialized made a clip-on for late model Venge. Don’t know how good it was or how low it got but it integrated with the Venge bars.

Are you planning to re-cable and have shifters on the extensions or just extensions? You could also look at low-stack shorty extensions, mainly for Olympic tris. Controltech makes one for example.

I had planned to use my Venge, size 51, for Olympic events and the only way to get low enough on it was a -25 stem and Zipp Vuka under-mount clips, at 37mm they were about the lowest stack out there. I never set it up because 2020.

Set up your road bike as a good road bike and your tt bike as a good tt bike. All you get with a road bike set up as a tt bike is something that kinda sucks at both. You'll have way more fun riding them both that way.