Don't think ERB ever wrote any Tarzan strips himself, but his son John Coleman Burroughs did write and draw a short-lived John Carter strip.

Doubt very much that Stout or Gardener wrote Nero Wolfe and Perry Mason strips themselves, but I suppose it's possible (I'm not an expert on those two). Not sure about Charteris and The Saint either.

Lester Dent wrote about a week's worth of sample scripts for a proposed Doc Savage strip, which were illustrated by Paul Orban, but couldn't get a green-light. The completed sample strips were recently printed in one of Nostalgic Ventures' nifty Double Doc reprints.