Can low durability be mitigated with specific finishes?
Short answer: yes, absolutely. Think of the long service life of softwoods used outdoors when treated with wood preservatives.
Is this finishing schedule enough to mitigate the poor durability of a wood like maple?
I'd imagine it will make a significant difference. Epoxy is an extremely effective water barrier, coupled with a continuous film of spar varnish liquid water is going to have a very hard time getting to the wood (note this won't prevent water vapour transfer, although it will slow it significantly). Without water getting to the wood decay is basically prevented I think.
Are there other finishes that can be used to protect a non-durable wood in the outdoor elements?
You can make your own penetrating wood finish for outdoor applications, where you add in ingredients (e.g. metal salts) to help prevent decay and/or fungal attack. In the old days arsenic or lead compounds were used for this both commercially and in homemade recipes; because of the potential toxicity issues with these I won't mention any specifics, despite their effectiveness.
In the modern world where we're a little more conscious of the risk of poisoning ourselves at home one of the simplest and most effective additives you could use is borax, which also prevents attack by termites and other boring insects. In the US at least this is widely and inexpensively available, sold as a laundry aid. Note for European readers: recent changes in EU regulations have made borax much more difficult to get here, it may now be impossible for non-commercial buyers to purchase in dry form.
In addition to being made into a solution with water and just painted on the raw wood as you would with any conventional wood preservative, borax can be brought into solution in oil.
As I understand the process you heat the oil gently (use a double boiler if necessary to prevent the oil getting to higher than 100°C / 212°F) and spoon in borax, stir to dissolve and repeat until no more will be absorbed by the oil and there's a slight gritty residue left at the bottom of the container. Longer heating may see more dissolved over time so that's worth testing out.
The resulting borax-infused oil can be used directly as a wood finish or blended with a varnish to make a DIY version of "teak oil", or with a natural wood resin (e.g. pine tar) which itself has some preservative effect.
Suitable oils for this would be any drying oil, principally boiled linseed oil or tung oil. Tung oil is reputed to give superior waterproofing but I've seen no direct comparisons of effectiveness. And if your plan is to blend the oil with a varnish then its superiority may be irrelevant as the resins in the varnish could provide all the required water-resistance.
