Could the Captcha challenge be restricted to those characters which are not ambiguous? For example, could ohs (O) and zeros (0) be excluded? Similarly, could uppercase eye (I), lowercase ell (l), and the numeral one (1) be excluded?
I realise that the numeric character is unambiguous, but that’s only OK if you see a 1. However, if you see an ell, then you have no way of knowing whether it is a one or an eye.