I'm interested, and in fact intend shortly to offer margin trading to large players and I'd just account for things manually. I'm interested in your platform and if it worked well, may consider licencing it from you.
What is there to stop someone from using this to create fake securities just to use your api to buy shares at 120% of input value, then run away with your funds, and their initial value?
Margin lenders deal with this by having an approved list of securities. They will often lend up to 75 % against blue chip stocks, but lower amounts for riskier assets. Penny dreadfuls generally count as nothing towards your collateral.