DSF

defiantly the cream would seep through the metal, as i have found out with vinyl being stuck to the glass also. i like your idea of something that can be reused but i think it will be a hard one to find.

Depending on the customers requirements if they just want the letters etched then I just weed the letters and etch through if they want the way your link shows I cut the size required then leave the letters in I've never tried the way of the link because it looks a right pain to do.

you can make reusable stencils for etching the ones I have seen are done with a UV exposure unit, Creative printers of London have one for £125 you can produce far more intricate detail than vinyl stencils and they say they are re-useable lots of info here http://glass-etching-kits.com/what_is_stencil.htm

Can you do different colour etching. ?

I don't know if you can still buy this but a friend gave me a can of coloured etch spray made by Humbrol he picked it up from a cheap store so it maybe something that was discontinued,I did try it and it worked but on the instructions it said it must be kept dry and away from sunlight.