The word "in" signifies that you and I are working together in the task of developing.
You might also say "Could I seek your assistance with developing a draft memo ?". This has broadly the same meaning, but suggests slightly that I will be doing most of the work and you will be helping me.
If the word "to" is used, as you suggest, then the sentence form becomes :
"Could I seek your assistance to develop a draft memo ?". The meaning is the same as when "in" is used, with the emphasis on the final goal rather than the work required to achieve it. This is a subtle difference.