uchristensen: ...I'd like to see a solution though. ;-)
And congratulations by the way. Now solve the other two! :-P
Thanks! Which two, though? Momo's and...?
Solution goes as follows. Since it was for Pi day, and decimal places, I figured digits from the decimal expansion of Pi would be important. The image didn't seem to have any hidden text or stuff, though I could have been tricked very easily there, in hindsight - didn't do my due diligence on it. Anyway, I figured at least some of the numbers in the code would refer to digits of Pi.
It quickly became clear that patterns of the form aX/Y had some meaning as a unit, though aX was meaningful as well (X and Y being natural numbers; Y = X+1 always, though the significance of that became apparent only after solving the whole thing). With those two hypotheses in place, the middle of the code started to look like two groups of 4 characters, separated by ~. It was then reasonable to assume that the ~ were simply replacing dashes (Obvious it may seem, but it wasn't quite so initially, when their placement appeared uneven).
The outlying parts of the code were trickier. I had initially assumed that maybe only #X denoted an index of Pi and I wasn't sure if groups of the form X#Y weren't meant to be read as a unit. With the middle structure in place, though, I figured I could still make groups of 4 characters if I read #X as a separate unit. Which left me with a nicely laid out code:
#9 a2 #2 a94/95 ~ a424/425 a94/95 a53/54 a4 ~ a11 a6 a13 a49/50 ~ #2 a95/96 a1/2 #6
Now, it became reasonably intuitive to assume that # meant digit and a meant letter. Also, if that was the format of the code, it meant probably all the numbers in it referred to indices in the decimal expansion of Pi. It was a question of whether the first index was 3 or 1, the first decimal. It turned out to be the latter, so #9 a2 #2 a94/95 became 3 a4 4 a1/1, however I initially thought the / denoted actual division (reasonable, as the first index being 3, offered most of the /Y to be 1, with a few X/2 that still yielded integer results; the first index being 1, though resulted in some fractions and division by 0).
At this point, I started trying out codes, but none worked. I was just about to write to Astrobogulator for a sanity check on my progress, when it dawned on me, that all the letters I'd been trying were early ones - because I'd been using aX alone, and X yielded just the digit of Pi at X. Once I tried to decipher X/Y as a two digit number, with the digits being the X-th and Y-th digits of Pi, the first interpretation became infeasible (3 being the first index) - no 82-nd letter ;) The other one made sense, though, and once I tried it, it worked. This also explained, with hindsight, the prevalence of the aX/X+1 units. In enciphering letters, Austrobogulator simply looked for an occurrence of their numeric representation inside the digits of Pi. So, to encode U, he would search for 21, not just any random instances of 2 and 1. This makes sense, as it gives important additional structure to the code, strengthening the implication that aX/Y is a single unit.
I should mention that in solving this, I had Lifthrasil's recent
Witcher giveaway in mind and salvaged and modified drennan's
Python script (very inelegantly, I might add) to find the right digits of Pi.
In hindsight, I feel better about claiming the prize, though I still feel I ought to have left it up for a few hours for people to try.
P.S. I had forgotten about this, but initially, when I thought that only #X referred to digits of Pi, I thought that aX denoted ALT+X unicode. The results were... compelling :P Let's just say that reverse Unicode could be one of the meanest cyphers out there. Or Wingdings. *shudder*