It is what is not owed against, but I can't get my bank to use any of it as collateral.  

Contract clauses were really a big help - principal only mortgage - that is NO INTEREST, installment down payments, money back, right of possession, many subject to offers, delayed settlement with possession - contract candy ( also known as buyer incentives) prime rate financing - money back from seller - discounting existing current and defaulted mortgages, early pay-off discounts, substitution of collateral (my favorite one)When there is NO equity - I offer --- Contract of sale (on a skinny), subject to existing financing, immediate possession, right to assign and release of liability -- This system works well when house lacks equity.

possibly use the homes I have under contract as collateral for the asset based funding?

If for any reason their loan isn’t paid, they would merely come in, take out their collateral, sell it and get their money back.They file what they call a UCC filing.

Remember that the basis of all loan underwriting are the THREE C’s: Cash flow, Collateral and Character.

Or if you have a good track record maybe you can get a business line of credit using the properties as collateral?

Can I use my lender note as collateral to get a bank loan?

They are lower interest than credit cards and the bank some has security because it's tied to your home as collateral.

What are you offering as collateral to the private lender?  

You may want to offer a conventional commercial lender additional collateral on a blanket mortgage (remember  to build  in a release of other collateral once the blanket mortgage is paid down to X %., this way you free up your equity to do other deals using the leverage in your existing equity)Also you might want to secure a tenant immediately, with a build out clause, secure the lease.