I had my fastback steering box rebuilt by a well respected Alpine specialist 25 years ago. He made up his own lubricant for them using a mixture of EP90 and bearing grease, (so effectively similar to your suggestion). I haven't had any problems or a leak since, (over many thousands of miles)!
Recently spotted some seepage from the steering box oil seal on my 1963 111A. Looks quite a faff to get the box out to renew the seal. Spotted a video on one of the Alpine forums (also on a US forum) suggesting that steering box oil leaks are pretty much endemic and suggesting that Castrol Spheerol semi liquid grease could successfully be used in the box instead of EP90 gear oil. Does anyone have any experience on this method?