I am pretty sure JL can allow access to code share bookings on their website if they want to, but they are not obligated to do so as it's marketing carrier/TA's responsibility to handle passenger's requests before the departure date. There was a similar discussion in the JL forum recently.

So JL gets a passing grade in this area. AA is doing better in this area in my opinion for their own flights. Though not always possible, I have been able to access seat maps of code shared AA-operated flights on aa.com. Of course, AA, as a marketing carrier of JL-operated flights, is doing terrible imposing HUCA for simple seat assignments.