I have done several times with MBB, but the highest was only RM16k, and that one I needed to write an e-mail and send proof of my balance transfer form.
Since then, my strategy has always been to keep each request below RM10k, which I believe is the amount they can authorise via phone without an e-mail.
For example, about 4 months ago I had a RM20k credit (from PBB) sitting on my MBB CC (after paying off the outstanding balances on all my MBB cards). I split this into 3 requests. I first asked for RM7k via phone, waited for it to be paid into account. Then I requested another RM7k...and repeated the whole process again. This may not be the "best" way, but it worked for me. They never asked questions about why I made so many requests. I think they deal with each separately, rather than looking at past history.
These few months I rarely have huge excess credits, because I use the "revolving BT" method - where I have several BTs all paying off each others' instalments.
Perhaps guy3288 can share his experience. His BT strategy is pretty impressive, and he may be able to give you better insight.
Advice taken. I just called just now for the RM30K to be spread out to 4 different cards involving MBB, HLB and PBB.