What items you planning on connecting? I don't know of any items that have AES output via a mini xlr. Are you looking to use the mini xlr outputs on the 722? The 722 mini xlr outputs are analog signals. AES output on the 722 is via a bnc connector.
My guess is that Simon is looking to use the AES output from his new Sonosax SX-M32. According to the manual, the AES output is via mini-XLR.
http://www.sonosax.ch/mixers/sxm32/sx-m32-umeng.pdfAnother way to go AES to S/PDIF is to use an impedance transformer. Like the BCJ-XJ-TRB 110 Ohm to 75 Ohm Digital Audio Impedance. I used to use this with my V3.
http://www.canare.com/ProductItemDisplay.aspx?productItemID=25however, to use it, you would need an AES cable (in your case mini-XLR to XLR), and then the impedance transformer, and then a S/PDIF cable (either BNC to RCA, or I always just kept a BNC-to-RCA adapter connected the whole time, which allowed me to use any RCA to RCA S/PDIF cable.
While that solution will work great, it is bulkier than mshilarious's suggestion to put resistors in the XLR end. of course, I don't know if that is still a viable option, because while it fits into a regular sized XLR connector, it may or may not fit into a mini-XLR connector.
Lastly, you could just ignore the impedance difference, and get a mini-XLR to RCA cable made, and not use the impedance transformer, not mshilarious's solution. While this is not ideal because of the impedance mis-match, for short cable runs (i.e. 1 to 2 feet), it probably won't make any difference.