I thought I would update this thread with the latest development.
Summary: I was able this year to use my SCB Easy 1.5% interest account to deposit my 800,000 and use that for this year's extension of stay at Jomtien immigration--so it IS possible to use this type of account even though it has no passbook.
Details: Last year, about a week after getting my extension, I moved my money to the SCB Easy account as I wanted to get the higher interest. I had a backup as I had no direct confirmation that this would be ok, but it turned out I didn't need it. I always do my extension as early as possible, about 30 days before it expires to give myself wiggle room to deal with any glitches.
I went to the branch of SCB on Soi 11? in Jomtien at 10:30 ish in the morning to get my letter and bank statment, which was 200 baht. The lady who helped me actually said to me that she didn't know this kind of account could be used for retirement extensions. I assured her that they had told me it could be used when I opened the account and that I was 98% sure that it could.
I had to wait about 45 minutes in the queue, so I didn't arrive at Jomtien immigration until after 11:30 am and assumed I would have to wait for 1 pm as I hadn't yet had my picture taken, done copies, nor filled out the form. I did all that at the copy place next to the bathroom inside immigration, and went to the counter about 11:50 where they were turning people away and telling them to come back at 1 pm.
I showed the guy my paper and he gave me a queue number! Nice, so I went to the counter and the extension guy looked at my queue number, then his watch and decided to help me. He went through the papers and everything was good but then he looked at the bank statement and said it was no good because no bank book.
He was amazingly helpful and patient. He explained that because there was no bank book, the STATEMENT had to go back ONE YEAR FROM THE DAY I APPLIED FOR THE EXTENSION and the one they had given me only went back a couple months. He then made a note on my package that would allow me to come back at 1 pm and skip the queue. I was so grateful, what a nice guy.
Another detail: as I had moved the money a week after getting my extension last year, the money had not been in SCB for a full year yet. I explained that to the guy, and he said that I could give copies of the bank book I transferred the funds from (it was a Bangkok Bank term deposit account with a passbook). I asked if I needed a letter from Bangkok Bank and he said no. The other option we discussed was waiting a week or so and using just the SCB account, but he said the letter from SCB was only good for 3 days, so that would require another 200 baht fee.
I rushed home to get the old Bkk Bank passbook, rushed to the bank where I got a more senior bank employee who knew the ins and outs better. She said I need to pay 200 baht extra for a one-year statement as they only give up to 3 months statements for free, so IN TOTAL I gave 400 baht for letter and one-year statement. She also copied my Bkk Bank passbook for me. That went very quick and I was back at immigration just before one--I was let in after a close examination by the door guy, bypassing a long queue. Phew.
After that it was quick and easy. A couple minutes tops, a few more signatures and 1900 baht, and I had my number to pick up my passport the next day, which I did. Now I'm good to go for another year. I hope this will be helpful to others.