Spit on it...
...in my experience, saliva gives you a greater grain recovery with the iron method than water does. The enzymes in spittle seem to do something "magic" to the wood fibres, particularly on "hard" species like maple. This was shown to me by an old Dutchman 20 years ago...try it and see what you think...
Let it "soak in" well prior to applying the heat; "flashing" with high temperature (cotton/linen setting) also helps, just be quick about it

. Multiple applications
will continue to increase the "recovery"...until you hit the "end of the line".
I've "gotten back" about 1/8" in Eastern Hard Maple in a few instances using this method, but it really depends on the peculiarities of the grain composition in the damaged area...
Don't know about injecting
this with a syringe though... never tried:whistling2:.
Cheers, Ron