Thanks to the six people who sent in answers to my question. All were interesting. I'll make a couple of more points.

First, have politicians interfered with top bureaucratic appointments (notably jimujian) in the past? Sure, they have a voice in who is picked among the two or three candidates within the ministry.
Moreover, a PM (e.g. Tanaka Kakuei) or a minister (e.g.
Tanaka Makiko) has intervened in a bigger way from time to time. But when they did, it made headlines, and often caused them various troubles. Moreover, most often the person conformed to the norm that the jimujikan is appointed from among the current bureau chiefs (or rough equivalent) in that ministry. When I say "most often" I don't actually think of any exceptions for actual ministries though there have been a few appointments in lesser agencies that did not conform to the organizational norm.

An interesting example is Abe's appointment of Muraki Atsuko as MHLW jimujikan. It was a notable and laudable appointment in that she is a woman, and that she had been unjustly arrested in 2009--she was then a bureau chief but the incident was when she was a section chief--in a scandal, and then exonerated when it was shown the prosecutor had faked evidence.
However, it was not that which surprised MHLW officials and observers of the bureaucracy. It was that she had originally been a Ministry of Labor official, and since the merger with the MOHW that created the MHLW, the jikan post had always alternated between the two groups of officials. She was the second MOL person in a row, which violated the norm and caused a lot of comment that Abe was taking an usually strong hand in such appointments. However, she still was a bureau chief when appointed so the main norm was not violated.

If a PM/cabinet/minister could routinely violate this norm, and in particular not appoint a "proper" official of that ministry (such as someone who originally joined a different ministry, or a businessman or something), that would be a gigantic change.

Second, can such a reform be passed? Some thought that bureaucrats have powerful means to fight back. I don't think so. Based on what? In defending turf, or some policy, a ministry (or bureau) may take the point for a strong alliance of interest groups and politicians (a subgovernment or cozy triangle). It is they who have the power though the officials may well have the ideas.
Power to protect the interests of the bureaucracy as a whole? Where is the evidence? Their salaries and perks have been cut consistently and they have been subject to constant abuse, and nobody stands up for them. Why now, with regard to how the personnel system works?

Third, if implemented would it make a difference? That needs a lot of discussion, and it would be difficult in the short run, but I sure think so, Read Bernie Silberman's marvelous 1993 book Cages of Reason: The Rise of the Rational State in France, Japan, the United States and Great Britain to see the struggles in the 19th century to depoliticize the bureaucracy and establish effective governance. (I reviewed it in JAS and will send that to anyone who wants a short summary of his theory about how the four nations differed.)

Fourth, would it be good or bad? As you may gather from the last paragraph, I don't share the view of many political scientists that the Westminster System is necessarily all that great, or for that matter that either the UK today or Japan as Abe would like to see it is really Westminster. It is more government dominated by the Downing Street or the Kantei aided by "experts" who are brought in from wherever. Looks more like the US if we could imagine the US without Congress.

I am not blind to how tatewari gyousei and other bureaucratic dysfunctions have often screwed up Japanese governance, but the alternatives are not necessarily all that terrific.

More comments welcome.

John Campbell