I would suggest you read these two cases:
http://www.leagle.com/decision/198043094Wn2d336_1397 (Cascade Bank v. Butler)
http://www.leagle.com/decision/198043094Wn2d336_1397 (Freeborn v. Seattle Trust)
If you read these cases you'll see that for years Washington law was somewhat of a mess when it came to dealing with the nature of the interest of a vendor and vendee in a real estate contract. Rather than ever adopt the doctrine of "equitable conversion" (converting the interest of the vendee to an interest in real estate), the court stumbled its way through to reach much the same result.
I suspect you could easily grant a bank a HELOC mortgage (or deed of trust) against a vendee's interest in a real estate contract. The question though is whether or not the bank would be willing to do that. Real estate contracts are somewhat more common in eastern Washington, so it's possible that this is common.
You probably should just contact one or more lenders to find out what they are willing to do. If you haven't purchased this property as of yet, you probably should also consult a real estate attorney, if for no other reason to make sure you purchase it with the proper documentation.