A novel notion, but the devs don't add a thing unless it helps THEM not us.
you have to consider:
does it make gold farming by sweat shops easier?
does it cause or relieve an issue that affects player attrition?
does it have unintended effects when combined with existing items?
does it present an actual advantage to play or is it just more complication to the code?
(i.e. does it add revinue or just cost more payroll and electricity to develop)
the reason you have to go back to the bank to begin with is to slow down the acquisition of gold. we werent ever meant to be multi billionaires.
if you want to carry 20k without affecting your weight then get a beatle.
a character with 150 str (which I have) with a beatle can carry 80k on the beatle and 25k in his pack while wearing 100 stones equipment. I get bored before I run out of room. You don't even need to have the strength like my fighter just take the beatle put 40k on it and ride home.
my assessment, it would be just another weird item a few people have that doesn't add any real joy to the game and is abused by the sweat shop guys so they can pick our pockets. the items for cash guys **** the vendors of the good priced items and drive the prices through the roof so that noone can compete then they want us to pay real money to order it from their websites.

I for one am against anything that adds more gold to these peoples coffers.
I SAY! stop all non intelligent creatures from dropping gold. no more sewer rat, slime, or mongbats. if it doesnt have pants it doesnt drop gold. When you kill a creature you should only get the weapon and armor it's wearing PERIOD. and a small purse of gold,maybe. I wish rares were still rare.
there is to much emphasis on huge party events and not enough celebration of the self made players. rant rant rant rant. sorry, off topic.
ahem coin of holding. nice idea, but is just a vain attempt at skirting the real issue, bags of sending currently suck.