First off, 'insitu' may be the wrong word for this.

But what I'm talking about is that Sodium Carbonate and Calcium Hydroxide combine to form Sodium Hydroxide.
We all can pick up Baking Soda at the convenience store and with a little heat that'll turn into Sodium Carbonate. Calcium Hydroxide is available as Lime at some shops or can be sourced inconspicuously over the internet (unlike Lye, which is a very sketchy chemical).

A proposed tek to replace lye would look something like this
-100g of MHRB is added to 100g Sodium Carbonate and 100g Calcium Hydroxide and mixed.
-Water is added and this is treated like an ordinary STB.

I've tried this (without the MHRB) by adding 1 tsp of Sodium Carbonate to 1 tsp of Lime and adding some water to all of that. A precipitate formed and was filtered out. The pH of the solution was measured at 13, which is greater than can be achieved with Lime or Sodium Carbonate alone. If you really want to you could probably evaporate the Lye-water but that seems like it would be unnecessary and dangerous.

With a STB tek the filtering could be skipped, which is a good thing because the lye-water has a tendency to eat through coffee filters.

Of course this isn't for everybody, but for those of us who are to paranoid to order Lye or don't want dangerous Lye crystals sitting around this is a pretty nifty solution.