If I understand correctly you have an image/graphic etc in Lab and you want to remove colours that are in Gamut for sRGB leaving the OoG colours available to you in some form.
Any Lab to sRGB conversion you do will complicate your requirement due to Intent where OoG colours will be put in Gamut using one of the methods allowed. (The sRGB that we currently use does not support Perceptual intent. I think there is a Beta version that does.)
I don't know the answer to your question but I would start my investigation using ColorThink Pro which is a colour tool.
It has a tool called a worksheet where when you load an image you can get an Excel spreadsheet of a sample of the colours in your image. The sample size can be very big! I did 10,000 samples of each of 40 of my images producing 400,000 samples which I then "played" with mathematically. You can get the spreadsheet in LCH mode and that is much easier to use to do what you want.
I would then sample the image and the sRGB gamut edge by slices in L and segments in H and basically devise a bit of Excel logic that looked for C in a particular slice and segment of the image to be greater than the C for the same L & H with sRGB. Those are the colours you want. They have a higher Chroma than permitted in sRGB.
The worksheet gives you the co-ordinates of those original samples so in your Excel be sure not to lose them.
CTP will give you an image if you format your sample data correctly but that may not be sufficient for you.
At some point you would want to display them and would need a large gamut printer to do so. There would be no point in attempting to print them or display them in sRGB as the software would attempt to put them in gamut which then makes all your work pretty irrelevant, however it still might produce an interesting image, just not what you described as your desire.
Whatever display technique you use again OoG colours would be moved in some way to make them visible. If you don't want that you would need to do something like the above again with your larger gamut to remove those OoG colours so that you were not "cheating" :-)
I think that turning the Lab colours and co-ordinates into a useable image is probably going to be the interesting bit of work. The stuff with Excel is just a bit boring.
I broke my pelvis and during my recovery I used my 400,000 samples to produce "Rex's Gamut", so it isn't a quick job.
I give a talk based on this work and one of my slides says "Don't do this at home"
Make sure your chosen image(s) are OoG in sRGB before you do all the maths! Gamut warning in PS can be set up to do that.
Best of luck. It will be interesting to see if there are simpler solutions.