Bill,
I didn't manage to find in Miliutin's work a direct reference to presence of engineer troops in Rimsky-Korsakov's corps. On the other hand in the combined army to be commanded by Suvorov in Switzerland a number of engineer units are shown: "a pioneers company, sappers and pontoniers squads". No numbers provided, as if they are not on the muster roll.
Regarding the headgear. The front piece was "english tin". Not silver, of course, but white metal. Your guess is almost correct. The classical work on the Russian uniforms is available online. The direct reference to the pages you need with pics is
http://www.runivers.ru/bookreader/book9 ... 5/mode/1up Images of different engineers troops are on pages 120-125:
Page 120 (the top one) A pioneer in 1797-1801
Page 121 (bottom) Pioneer headgear (top) Sapper headgear
Page 122 (bottom) Pioneer NCO and Drummer (top) Sapper
Page 123 (bottom) Pioneer regimental drummer (top) ober-officer of pioneer regiment
Page 124 (bottom) Pontoniers (top) Pontoniers in working outfit
Page 125 officer and NCO of pontoniers companies
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Leib-Guard Cuirassiers Regiment's
General-Fieldmareshal Count Anton Kosyanenko
Commanding Astrakhan grenadiers regiment
2nd Grenadiers Division, Russian Contingent