I can't even figure out how to offer someone else's trade power.
You need to ask for a nation to transfer trade power to you. You can then transfer their trade power in the peace deal.
Even if you only get the minimum 10%, the established transfer will be a full 50%.
You can test this easily in a burgundy 1444 start, before unpausing declare a war of reconquest for gelre, austria will join.
Step 2, ask for trade power from just about anyone nearby. Utrecht gave me 10% of theirs without a fuss just like that.
3, ask to give utrechtian trade power in the peace deal asap one month after war declaration, and it will count as a 60% WS cost.
The UI will tell you that they do not want the trade power, but they will accept it without any fuss.
An offer worth 60 will beat the length of war modifier of 44, and also the alliance and military power differences even without a single combat, without even standing on gelre.
Repeating the war before, i ask for frisian 10% trade power and attack liege for a claim. I need to step on their 5-stack to wipe them to get the instant peace this time, and BAM austria gets another 30 IA.
Austria gains IA for defending the empire and will enact the first reform exactly one month after the 1444 start. They will keep getting 30 IA one month after every burgundian claim on the HRE is fabricated for as long as burgundy wants to. As austria in my game i have been racking my brain over how to get foreign nations to attack the HRE, but the answer is obvious. You just need to start outside the HRE and ninja the throne once it is ripe.