IMO this is not valid collateral.
What is considered collateral?
Collateral is something that can easily be resold to cover the loan value plus interest should the loaner default on the loan.
- The best collateral is another crypto-currency, such as Litecoin (also written as LTC). Coins must be moderately traded on multiple exchanges.
- Some digital wares such as domain names can be considered as long as the user cannot recover it.
- Small valuable items that can be shipped though the mail - i.e. gold, silver, iphones, etc.
- Large items, such as a motorcycle or guitar can be used if you live close to the person giving you the loan. Try localbitcoins.com
COnvertible notes as you said are only really worth anything if the start up is successful. And plus, you'd have to wait quite a while actually to get the cash into your hands. A collateral must be readily liquidated in the event of a default.
Domain names that aren't 4-5 letters .com domains aren't really accepted as collateral either, of course it'll depend on your domains but i highly doubt it is anything valuable.
I recommend you just getting a personal loan from your bank, or an overdraft. You'll find much better interest rates and much higher rates of acceptance.