A few things:
1. There were collar insignia in Luftwaffe that could be 'navalized' by adding the wavy lines, anchors, and/or device specific to the corps the officer belonged to.
2. There appears to be a way to 'navalize' the collar insignia for the Wehrmacht to form collar insignia for
both the Navy and the Wehrmacht. The sub-, junior, and senior (and general, but that's less important)officer collar insignia were distinct. Simply add a rosette or two, and you get an insignia that would take into account the shoulder boards of a lieutenant and major. Somewhere I saw a 'soldier' with gold instead of the silver highlights. I'm 99% certain he was a sailor or a coastal artillery man. If that were the case, then using silver for the Wehrmacht and Gold for the Kriegsmarine would be plausible.
http://www.uniforminsignia.org/?option=com_insigniasearch&Itemid=53&result=370
http://www.uniforminsignia.org/?option=com_insigniasearch&Itemid=53&result=367
http://www.uniforminsignia.org/?option=com_insigniasearch&Itemid=53&result=2822
http://www.uniforminsignia.org/?option=com_insigniasearch&Itemid=53&result=380
On to the Imperial Japanese military.
According to the 'Imperial Japanese Army and Navy Uniforms & Equipment' NEW REVISED EDITION [sic] by Tadao Nakata and Thomas B. Nelson:
pg. 118:
IJA, Flying wear, winter, new style, old style.
Two relevant pictures of the above are shown with the IJA rank tab sown to the upper left arm, between elbow and shoulder, only.
pg 119:
Flying wear without life jacket, as before but with life jacket, and flying wear summer.
Four pictures with rank tabs on both collars.
The book is in Japanese, but the one I got came with a pamphlet with descriptions of the photos. I copied the descriptions nearly, but not, exactly.
The first 100 pages or so has rank insignia from various periods, so I'll see about scanning (or at least taking pictures) and sending them off to the sight.
Now, if only I could find that frelling Kriegsmarine book I have.
edit
http://img12.imageshack.us/img12/1958/kriegsmarine.png
Top row is based on the Luftwaffe. First two are enlisted, remainders are the top ranks in the Junior, Senior, and Flag Officer categories.
Bottom is inspired by the Wehrmacht. First two are, again, enlisted, while the next two are the lowest Junior and Senior Officer ranks. The last one is for K. z. S.
Using the gold for navy, silver for army, collar ranks could be assigned. I did not include the collar insignia for general officers and field marshal, because I've been up for too long, so it would be too much trouble.
. The FM could use his crossed batons or the special collar patch since it's unique, while generals would use their tabs plus an increasing number of rosettes. Not sure how that would work in the navy, though.