Tie it to the random-slow (and hurt Ashe's identity) Apply True damage to every 'not 1st hit' (and risk a Marksman with a consistent source of True damage for most of the game)
In other words, one option is for enemies who are afflicted by frost arrow to be considered "critically struck" when hit by Ashe for the sake of Infinity Edge
or
*Apply True damage to every 'not 1st hit', but only if she also has a +Crit item (which is slightly non-intuitive to how Ashe works, and may be tricky to code and cause bugs)
apply Infinity Edge's true damage to enemies afflicted with frost arrow if she has a +crit item.
You do realize the only difference between your two "options" are inconveniencing Ashe to buy Brawler's gloves, right? Cuz at that point she would have "+crit". 400g separates your two options.
And if you mean the chances of proc'ing the true damage is related to her critical strike chance, then you're in the same boat as me except I'll be generous and even if it's her first auto attack that crits, she'll get part of that as true damage (it's not like her first attack does crazy damage if the target doesn't have the frost arrow debuff).
I'm honestly fine with Ashe's auto being partially converted to true damage if she gets the empowered slow on her auto. Wanna know why?
Here's literally the word for word quote of Ashe's passive in the client.
Ashe's attacks slow their target, causing her to deal increased damage to these targets.
Ashe's critical strikes deal no bonus damage, but apply an empowered slow to her target.
And I get what you're trying to argue for; Ashe is supposed to be the pillar of sustained, consistent damage through auto attacks. But that doesn't mean she can't buy a shiv.
This isn't about the champion. This is about the item and yes, if infinity edge works the way I describe she'll get slight spikes in her damage due to some of the damage being converted to true on a critical hit, but it's not like she's getting spike damage comparable to a regular ADC.