If you've spent any time at all with the Saiga12, you'll quickly realize that the "last round bolt hold open" concept is a huge pain point. Specifically, the fact that a loaded magazine is nearly impossible to load into the shotgun in a reasonable amount of time. Thus, the idea of an AR-style bolt hold open - and bolt release - makes sense on this shotgun.
The Russians are moving that direction, in that they have a magazine feed well for the latest version of the "police model" Siaga. The other company - VEPR - also has a bolt hold open device that is activated by the follower, similar to a PSL or a VZ-58. There are some very clever 3rd party bolt hold open devices as well, although they usually require some sort of modification to the magazine.
No one has - as of yet - developed a good bolt release mechanism. I intend to make one for my S-12. It will work in a method similar to the AR, however I do not want to drill holes in the outside of the receiver, nor do I wish to heavily modify the action to make it work.
I have experimented with a release arm linkage directly attached to the (factory) bolt hold open lever. The pressure required to operate it is too great for it to be of any practical value. The main spring is simply too powerful to overcome with finger pressure.
I believe it is a question of torque.
My next attempt will be to work out something similar to the pic. The dashed lines represent the factory bolt hold open, and the magazine follower. Front is to the right. The solid lines represent the bolt release conceptualization. The idea is that the lever in the trigger guard will be pushed downward, and it will rock slightly forward. This will engage the "release arm" that is attached to the trigger axis pin. The cam action should provide enough leverage to overcome the tension provided by the main spring.
This "release arm" will have a long activation lever, and a short action lever. You can't really see that well in this pic, so bear with me. This short arm will engage the factory bolt hold open, pushing it downward, thus releasing the bolt to spring foward.
The other part to this is the step-shaped bolt hold open. That will be nothing more than a PSL-style bolt catch.
The only unanswered question is how to link that to the factory bolt hold open, or how to re-arrange the bolt release arm to disengage the PSL-style bolt catch, so that the two parts are connected. I think there is enough room inside the receiver to play around, so there may be a simple way to do it by simply redesigning the "release arm". As it is represented in the pic, the "release arm" will disengage only the factory bolt hold open device.
I will have to make some more modifications before I can be sure this will work. If it does, then it should be able to be adapted to the existing 3rd party bolt hold open assemblies.
An AR-style bolt release tab (the one on the left side of the receiver, above the magazine) could still be installed. I just don't want to go that route unless I have to.