Hi David

Just for interest I carried out a simple experiment and found that a length of braid on braid increased in weight by 50% when I ran it under the tap to stimulate a reasonable rain shower. It was not soaked through. I did this partly because I noted how heavy all the external halyards were when I took them off the mast for the winter. That's perhaps why people have all internal halyards. 

A number of Twisters, including me, have an extra inner wire forestay. This normally sits lashed back to the shrouds but can be deployed and fixed to a U bolt aft of the forestay. It's tightened with a rigging screw [with a handle]  that fixes to the U bolt with a pelican hook. I have never used the setup in real desperate anger. I should have done so once but survived with a well rolled up genoa. 

If you search the Magazine Archive with the term 'inner forestay' you will find three articles.  

Yes, the jib does need support. Yes, a quick look at the catalogue tells me you are going to need a substantial block to take the load. I was mystified, last summer, by some bits of plastic on the deck...and then found that the block on the kicking strap had failed. The sheave had collapsed but the rope was still sort of going through the block. So, not a catastrophe.

You might also want to think about chafe on the Dyneema. I'm thinking that this might be the biggest issue.

I'd also note that Twister of Mersea has a set of tracks inboard of the shrouds that are, I much believe, for a small jib for heavy weather windward work. 

It's worth a try though.