Ken,
There is a little bit of good news and a lot of bad news. The good news is: Yes it can be done.
The bad news is: It won't be practical. IEC has one corner frequency of 4500Hz, while RIAA has three corner frequencies 50Hz, 500Hz and 2122Hz. That means that you have to do four different compensations to make it flat. To do a reasonable compensation for the 50Hz corner you will lose 10dB (similar to the NAB to IEC converter), however to compensate for the flat area between 500Hz and 2122Hz, you will lose another 13dB. So, the total loss of the passive converter circuit would be at least 23dB.
A second, even more important problem is that the exact frequency characteristic of a repro head is not universal. It differs quite a bit between manufacturers. The R2R manufacturer needs to compensate for the head specific deviations in their repro electronics and melt in the EQ at the same time. So if I would design a RIAA to IEC converter circuit for a Revox head, it would not work correctly with your Otari and it would again be different for your Teac.