Please report back! I havn't needed to do it for ages and it certainly worked on what must have been Victorian or Edwardian putty. Most of my windows now are Critall and the putty just falls out.
Ordinary non carbide round saw teeth cutter will do and not make ground glass waste.
In the particular job I was thinking of (c1860's Church, small stained-glass vent panels puttied into steel surrounds), the 'saw tooth' multitools lasted about a minute before losing all of the teeth... and it wasn't from hitting the steel surrounds, it was something very abrasive in the putty itself.
With the carbide tool, and a bit of finesse, it was possible to cut a groove in the rock-hard putty without doing too much damage to the lead surrounding the panel. A stripping blade in the groove could then be used to lever out chunks of putty, and the same knife, inserted from inside the building, was used to free the panel.
If it was conventional glazing (no leadwork) then I guess there's a danger of producing ground-glass 'grit' - but you'd need a mask and eye protection anyway, with the putty dust..
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