How important are capital requirements for controlling broad money supply?
On different banks, and at different times, different regulatory constraints may bind on their ability to create money: 


Reserve requirements relate bank lending (bank assets) to the quantity of reserves held by that bank at the central bank. 
Capital requirements relate bank lending to the amount of equity capital of the bank.
Liquidity requirements relate the asset market liquidity of a bank's assets to the tenor of a bank's funding.
Stress tests attempt to model the liquidity and/or solvency of a bank under an adverse scenario that affects the value of assets on its balance sheet and/or its funding.


In today’s environment, capital requirements and liquidity requirements are the constraints most likely to be binding on US banks, though stress tests were also binding on some banks in the recent past. Reserve requirements don’t currently bind in the US, as banks are awash in reserves, and as noted in the question, many other countries do not have these requirements. 

The fact that capital requirements are a brake on money creation is quite straightforward: if a bank that is constrained by capital requirements wishes to lend more, it must first raise costly equity capital. As a result, capital requirements are by definition a structural constraint on money creation.

Having said that, capital requirements are not a tool that one uses primarily to control the money supply. Instead, they're used as a prudential measure to ensure the solvency of banks. This is because, in the framework of instruments and targets, capital requirements are a very poor instrument with which to affect either of the two main targets of monetary policy, inflation and unemployment. 

They're a poor instrument for these purposes for at least three reasons: (1) the relationship between capital requirements and money creation is in no way linear— as noted before, capital requirements are binding on different institutions at different times, so it's difficult to predict the effects of tightening or loosening them; (2) changing capital requirements has a lot of consequences (such as the potential impact on bank solvency, and cost to banks of raising additional capital or shedding assets if a tightening of requirements puts banks over the new, lower limit); and (3) capital requirements affect only banks, while over half of financial intermediation occurs outside banks.

Conversely, interest rates are pretty good at affecting broad money supply, and in a really efficient way, as marginal lending opportunities are compared to the price of money. At the same time, interest rates are pretty useless as a prudential instrument, as demonstrated in the early 2000s.