Water level detectors are cheap (http://www.argos.co.uk/static/Product/partNumber/9246470.htm?storeId=10001&referredURL=http%3A%2F%2Fwww.argos.co.uk%2Fstatic%2FProduct%2FpartNumber%2F9246470.htm%3FstoreId%3D10001&jspStoreDir=argos&_%24ja=tsid%3A11674|prd%3A2010640&_%24ja=tsid%3A11674|prd%3A1546795&referrer=COJUN&cmpid=COJUN%23pdpFullProductInformation). Definitely viable.

My M-i-L has one that sits on the side of the cup to measure the water level and beeps. Very funky.

http://www.rnib.org.uk/shop/Pages/ProductDetails.aspx?category=utensils&productID=DK12701