Can you convert any 29er to a mullet.

Head angle, seat post angle, BB drop and pedal clearance.

I've just built a Jeffsy 29er up as a mullet for a mate, I've put the shock in the higher position but he mentioned he was getting shorter cranks for it last time we spoke, seems to like it so far tho

another consideration, I had a sonder signal TI (hardtail) and unless i ran slim rims and tyre, the tyre wouldn't fit between the CS yoke... it was shaped further forwards to wrap around a 29er... 

some frames have quite a heavily shaped "tyre bit" for want of a better word, you might just find that a 650b tyre of any useful size doesn't fit in the frame, my Nicolai is like that. 

Other than that BB drop is probably the biggest potential issue, it's why mullets work so brilliantly with older bikes- they tended to be taller. 

Basically despite some total bullshit from manufacturers, unless there's adjustability or you're going with a proper plus tyre there's no geometry that's "right" for both, it's either too high for 29er or too low for 650b. But that works both ways and even now a lot of 29ers run a little high- my Remedy 29 was great but it was pretty tall in BB and there was nothing to be done about that, except for mulleting, it was never better than with a 2.5 650b rear. If I did the same with my Bird I think I'd pedalstrike myself to death.

One of these? 

https://www.williamsracingproducts.com/shop

As Northwind suggests, some modern bikes just have too low BBs for it to work.

You can convert any bike to a mullet, but that doesn't mean you should!

I have a 29er Vitus Escarpe that I am currently running as a mullet. A few pointers about the bike that made this worth a go. It has pretty short chainstays, that 29 rear wheel is tucked in pretty tight and the chainstays are not much different to many 27.5 bikes. It also has a high/low shock position.

So initially I put the shock in high and popped a 27.5 wheel in. At my local trails it was slightly less good on the climbs and slightly absolutely more fun on the descents. The problem I had was that, even with the frame in high I was still touching on the pedal clearance danger zone. It was a 6mm bottom bracket drop which doesn't seem like much but it is noticeable. I later took my fork up another 10mm which pretty much cured the pedal strikes. 

I buggered about with the saddle position to improve some of the climbing issues, the lower rear had moved my body back while seated. Then I needed to do it again when I added more travel up front. 

Overall does it make it a better bike? No. It is just different now. But it did confirm a few things about what I like in a bike so it was a worthwhile exercise.