Disconnect it from the 7028!
If the 7311 was bought in the first place then there must be some required resources in it, so disconnecting it and throwing it away is probably not an option.
As long as it is connected, the system will detect it on boot so it will checkout the resources and find the dead PSU.
In the D20 the second PSU is an option, it is not required, so you can just replace it with a blank.
What are the details of the problem / error and or diagnostic results? Are you sure the PSU has failed?
Are both D20 PSU's pugged in, or did the cleaner unplug one to plug in the hoover?...seen it many times.
Option 2:
Write a script and put it in a cron job to clear the wall rc.powerfail message from cron.
The diagela cron job usualy runs at 04:00, so at 04:01 run a job / script to remove the entry...but then you won't know if another power problem happens, then again would you anyway if you always get the message for the D20 walled.
Option 3:
Get in before they do and run diags, if all is OK (apart from the D20 PSU) remove the wall cron job.
Option 4:
Buy a PSU or a blanking plate from ebay / IBM reseller / IBM part sales and talk a person on site through replacing it. This is probably the cheapest option and all the info you and they need is on the web / in the service books - and they are hot swap:
http://publib16.boulder.ibm.com/pseries/en\_US/infocenter/base/hardware_docs/pdf/380627.pdf
Page 150 would be worth a read.
Page 168 gives the part (fru) numbers for the PSU and the dummy PSU filler.
A web search for IBM part sales should give you plenty of places to ask about prices.