It's been a lot of years
In my newsletter article, as near as I can recall, I had the Poles (a) Retain the ability to read Enigma. They could do it in 1938 both through their own efforts and through a spy in a crucial position, but lost both avenues before the German attack. (b) Develop effective 'bazooka'-type anti-tank rockets, (not too out-there--historically the French had an anti-tank rifle grenade based on similar principles ready to go into production in the summer of 1940, though apparently none actually got to the troops (another what-if) and (c) Not lose a brilliant aircraft designer who was historically killed in an air crash. As a result he goes on to develop a reasonably modern low-wing monoplane fighter, which goes into production in time to get a couple hundred in service before the war starts.
I also did a related scenario where the Poles develop a light-weight, very maneuverable fighter comparable to the Zero and get a couple hundred into production. Result: Until the Germans adapt and the Poles run out of pilots, the Poles slice through German air formations as easily as the Japanese sliced through veteran Spitfire pilots early in the Pacific War.
In terms of this challenge, I suppose the reading Enigma bit would fit. Knowing that the Germans were going no matter what the Poles did would help the Poles get troops mobilized and in place. Knowing that the Germans were planning to send a major force through the Polish Corridor and then attack from East Prussia would help. I don't think that alone would do it though.
Some brainstorming ideas:
- The Japanese decide to escalate Nomonham and as a result the Soviets decide not to do the pact with the Germans, or the Germans decide that they don't have to. As a result, the Germans have to conquer all of Poland. They could do it, but it would take longer and their ammo supplies were pretty close to depleted by the end of the historic campaign.
- The rainy season starts early. Parts of Poland are like parts of the Ukraine in terms of nearly impassible mud. Historically the fall rainy season came late in 1939. If it had come early--say September 10, then things could have gotten interesting.
- France works out a mobilization scheme that can actually get their forces mobilized and in place for an attack by September 5 or 6, or starts mobilizing early enough to get their forces in that kind of position.
- France works out their production problems with the engines for their MS406 fighter planes and as result fills the Polish order for something like 160-170 planes before the war breaks out (and has enough planes left over to mount a credible threat in the west). The MS406 was not a particularly good plane, but it was good enough to take on early model 109s and could catch up to most German bombers, which would force the Germans to escort their bombers, thus tying up fighters that would otherwise be strafing and otherwise messing with the Poles.
- Poles seize a hunk of Czechoslovakia, or somehow get part of the Czech army to retreat into Poland to be interned (thus giving them a hunk of the Czech arsenal and denying it to the Germans).
Doubt if any of these would meet the challenge, but then again it isn't an easy one to meet.