Could my Bht.800M seed money on a passbook account, requirement for extention of stay based on retirement, be used as security for a credit card with Thai banks?
Do any of the Thai banks offer secured credit cards?
If any of the ThaiVisa members carry such card, please share the info on the process of obtaining one.

Bangkok Bank will sort you out, I have to keep a set amount in a passbook account, the credit card is tied to that account, my CC charges are made to my savings account, I have 100,000 Baht limit and 200,000 in the passbook account.

This is a guess, as I have never seen your question discussed here. I don't think that Immigration would know if part of your B 800,000 was pledged to the bank to secure your debit card. The only way they might is if the bank stated such in their letter.

If you request the bank to write a letter stating that you have had the requisite amount on deposit for the requisite time, I think they would write such and stop there, without providing any other information about you.

This topic was recently discussed here on TV within the past month.

I bank with BB. When applying for a credit card, BB asked for 100k to be deposited in a separate deposit account in addition to my savings account.

When applying for a 1-year extension of my retirement visa in Dec 2014, Immigration looked at my savings account plus bank letter for proof of 800k baht. They disregard the credit card 100k. That is, in my case the 100k credit card deposit account did not count toward the 800k requirement.

I have 200,000 with Bangkok Bank in a long term savings account that secures my credit cards. I was told at one of the branches that I can use the 200,000 but would have to surrender my credit cards.

The bank has included the 200,000 as part of my 800,000 requirement in past years.