In principle, you could just use a simple orifice to act as a flow restrictor. The IP on the first stage is fairly constant, so all you need to do is start with a small hole and slowly increase the size until you get a flow rate of 12-15 liters per minute.
The rest of it is just a matter of going from the 1/4" NPT thread on the downstream size of the LP QD to male 1/4 NPT adapter to the restrictor to a barbed connector to attach to the hose to a bag type constant flow O2 mask. The restrictor itself could just be a thin round metal plate (with a suitable sized hole in the center) cut to the right diameter to fit snugly inside the female 1/4" NPT to 1/8" barbed connector fitting you'd screw on the LP QD to male 1/4" NPT adapter. The whole thing would be maybe 2.5 " long when done.
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Constnat flow O2 regulators are cheap, but demand regulators are not. If you use a small O2 bottle instead, you would want to use a demand regulator. Alternatively the cheaper constant flow model works, but you need a larger O2 supple - an E or Jumbo D cylinder, or better yet two of them to give you an hour or so of O2 at 15 lpm.
The attachment for a deco reg makes sense as you are not carrying dedicated O2 tanks around as extra baggage that you will probably never use. After a while, you'll leave it home and that is where it will be when you need it. A mask and small adapter would weight maybe a pound and would fit in a pocket in the dive bag, so you'd be more likely to have it in the event you needed it.
