I remember a while back Archie or Urby talking about decals in GoldSrc, and I think the general consensus was just manually editing your texture was the best, fastest way to get the wall adornments you want in the game. (If you really still wanted to use the ugle HL decals, I suppose you could edit/copy/paste them over your textures as well)
In a limited way you can make signs and texts out of brushes--especially using Sledge's Text primitive--, though this won't look the greatest or be the most efficient from a resources perspective. If you opted to go this route, make sure you use Sledge or a HLFix-enabled Hammer, because it will allow you to make the brushes sub-1-unit thick, and/or nudge them at sub-one unit distances (Hammer compile by default snaps everything to the 1-unit grid)
Source SDK of course has overlays, which can be rotated/scaled every which way