If the cost of buying the correct line is too expensive then the cost of buying the wrong line and later replacing it with the correct line will be doubly expensive. 3.5% stretch with a 50kg load is 21 inches on a 50 foot halyard.
OP here, again. Will try to summarize.
Thanks to ALL for your insights. I have learned a lot from this thread and it helps me to make a better decision. I also did some amount of additional googling. One well-written analysis is by Beth Leonhard and Evans Starzinger. It may be of interest to some others, so here is the link: http://www.bethandevans.com/pdf/rightlinejob.pdf
You have convinced me that using this arborist rope for my main/jib halyards would not be a good idea. However, I will still buy a couple hundred feet for other purposes:
One is for a spi halyard. As Beth and Evans point out, some stretch on that is even advantageous, to deal with shock loads. The same applies to the mainsheet and the traveller lines (although I obviously won't need 100' for this purpose).
Another application I have is for a spare halyard. Last time I had the stick down I installed a block at the mast top as a spare, just in case. I have never used it and I may never need it, but boy would I be glad to have it if I did. In such a case, I doubt that I would worry about a couple inches of stretch in the line.
What I don't know is whether this line will prematurely degrade with exposure to salt (not so much an issue here on the upper Chessie) and UV. This is a risk I will take.
So, that's my plan. I will scratch the idea of using it for jib sheets or main/jib halyards but there seem to be viable applications. And I will save a sizable fraction of a boot buck doing so.
Again, thanks for the informative discussion!