Scoring for "look alike" characters in subject?

[Steve Dondley]

I'm noticing a fair amount of spam getting through using letters in the 
subject line that are outside the standard set of ASCII characters in an 
effort to bypass spam filters. For example, instead of a capital "R", 
there will be a letter that closely approximates a capital "R" but when 
you look closely at it, you'll see the bottom of the rounded part of the 
"R" never connects to the line running along the left side of the 
letter.

I don't want to use a rule that is too-restrictive (like maybe banning 
all non-standard ascii characters) but I also want to increase the 
likelihood of email using these tactics getting flagged as spam.

I'm new to spamasssassin so I'm not sure if a rule like this already 
exists or how I might go about finding this rule or what I should weight 
it. I'm wondering if others on the list have rules to address this same 
issue and can share their rule. Thanks.