I get asked this question all the time - and have never found a good answer. Linseed as you have discovered, is very short lived outside.
A glazing contractor told me this morning that he has been using somthing (he couldnt recall what), on his oak glazing cover strips, that Ronseal had recomended - you could try them, - it stabilises the timber but still allows it to turn grey under/as a result of UV - this was the end in mind for him.

Most things look nasty, but Charltons Timber 01373 812 501 did have some sort of oil that they thought would work, you could try them.
Good luck and please post any possitive results on this page