What do you propose to do with this information?  For now, you can include it in the notes field for the account, but I assume that is not really what you want.  Just adding a field for interest rate shouldn't be too hard, but I don't see the point.  I also don't know of any way to automatically download it, as I've never seen it as part of downloaded data for any account I have.
Also, remember, what KMyMoney calls a "cash" account just represents the cash in your wallet or cookie jar.  I assume you are referring to a bank account (checking or savings) other than an investment account.  However, it could also apply to a credit card account.
Can you elaborate a bit?

Thanks for the prompt reply!  

Yes indeed you can record the rate in the comments which is what I currently do but it's very messy for simple reports however such as a list of account details. You can't easily parse the rate from the rest of the notes (technically you could using special characters etc but hardly a newbie workaround) so a report must include all the notes (which may take up half the page with) just to detail the associated rate. Mixing it in with notes also precludes using the rate to calculate expected returns on fixed term accounts for example.

I agree that you can't download it easily but it one of those useful facts that doesn't change so often you can't update it manually but it does vary (more often now) over time on savings accounts, credit card accounts and loans so it's useful to be able to quickly see and compare rates across accounts in a list.

I thought an extra field on the account record would be a useful feature for many users and no so onerous to implement. 

You're quite right I should have stuck with the more precise account terms I used in the title rather than shorthand-ing them as "cash accounts". 

Thanks again for your "interest" ;-)