Is it possible to specify a default change address? Is it possible to enable default change address on my bitcoin-qt (aka bitcoin-core) client?

I'm using a webservice that monitors my balance (based on public keys) and whenever I send a transaction my balance goes to 0.0000 because my whole funds end up in a random change address.

I know there is a coincontrol option to specify a change address, but sometimes I forget to enter an address. Is there any way to configure the client to use a default change address? Where all changes end up?
Yes you can. Although it is discouraged.

Place the following in your bitcoin.conf file

changeaddress=yournewchangeaddress

Unless you have a real need to keep the address count in your wallet low than you shouldnt use this as it opens up security risks and destroys anonymity.

You should be using a new address for every transaction you make.