Eric is entirely correct on all fronts, slow cooling gold, silver (and copper based metals to some extent) will result in the growth of large crystal structures that will behave more like work-hardened material and break easily. I follow this rule all the time with copper when rolling staples for bagpipe reeds. The opposite is true with steel alloys, in general slower cooling is better. As Eric pointed out, engraving is unbelievably difficult. I've tried a bit too and realize it will take a lot of dedication, if even that. Very little room to cover your mistakes. And the helmet would need to be annealed. I second Eric's endorsement; the book Tim McCreights "The Complete Metalsmith" is a great book for anyone interested in artistic metal work, not just jewelry making. Highly recommended. So there you go Eric was right on all accounts, listen to Eric he is wise!
Back to the question of inlaying the M-1 helmet. You could engrave on the hardened steel with a rotary tool or die grinder, not sure how clean the lines would be, certainly not as clean as engraving. I know that I would certainly screw up the design with said tools, but your mileage might vary! "The Complete Metalsmith" describes ways to etch steel, I have never tried it but I think that knife and sword makers have been using this technique for a long time.
Another approach might be to simply cut your patterns out of brass sheet (something a little more thick than leaf). You won't be able to get the intricate small patterns like with damascene, but you could add some shiny bling! Brass will polish up to look like just like gold, varnish will keep it shiny. I work with 1/32" thick (22 gauge) brass sheet for making ferrules for bagpipes a lot and it can be cut with a good pair of shears, and of course it can be sawed easily with a jeweler saw (or scroll saw if you have access to one). Once shaped to fit the helmet, it could be attached with adhesive or, preferably rivets once you are sure where you want it. The riveting process requires you drill holes in the helmet, but beyond that it is a forgiving process to learn and you can screw up a few times and try again to get it right. Done correctly, the rivets can be filed flush and will be invisible. I do this all the time for attaching springs to metal bagpipe keys, smooth as butter. If you used thinner sheet it would be easy to cut out and look almost like it was inlayed if you bevel the edges by filing and sanding.
Anyways, sounds like a great idea, and a lot of fun. Working metal is really fun, you'll be hooked!
I would not Manganese steels are ductile when annealed so it is possible to soften it and engrave.