found the CXA1645M on eBay:
https://www.ebay.com/itm/-/200760207060
(ended up using the CXA2075; bought a bunch off of AliExpress)

general-purpose RGB-to-component converter:
http://us1.webpublications.com.au/static/images/articles/i1026/102648_2mg.jpg
(linked at https://damntechnology.blogspot.com/2008/08/apple-iigs-rgb-video-to-component.html?showComment=1218749040000#c7514385666987320654)

an RGB-to-S-video converter based on the CXA1645:
http://www.nexusuk.org/projects/rgb2svid/

info on the CXA2075 by one of its designers:
http://www.videoi.com/~pietro/cxa2075/

