I'm not exactly sure how fire resistant the "Trex" or "engineered wood" decking planks are, but I'd guess moreso than regular wood. They're at least the same thickness and look of your current wood planking. Just an idea.
Only other thing I can think of is to perhaps cut out and remove the portion of the wood planking that you want to replace, use a marine grade plywood (read water resistant) that would be thinner than the current decking planks, then lay in dry fit tiles or pavers or something. You could drill holes in the plywood to get rid of collecting water after rains.
I assume you have the 5/4" boards (which have probably dried/shrank to a pretty consistent 1" thick? If that's so, then you could use 3/4" marine plywood, then lay in 3/16" terra cotta tiles or something like that. Just lay them in there dry, then you can remove to clean if you want.
OR, you COULD actually use thinset mortar or mastic to actually adhere the tiles to a substrate and then grout in between them if you want to go to that extent.
Just my first thoughts.
18.5", 22.5" OTS
Large BGE (chambersuac's)
The Fiery Furnace (chambersuac's)
Big JT's Smokin' BBQ Competition Team