If you are certain there is no way to have what I suggested with the Marshall models with XLR out, or be able to rent a Line 6 DT50 Head - that's a shame.  BUT I still think the way you are describing what you wish to do is not the best way to handle this situation.  Too complicated and too easy for something to not work correctly and produce sounds on stage or through the PA that you would not be happy with in my opinion.

If you must go only from the board, and miking the amp is not an option either - which I do not understand why that would be a very simple solution??

However, I then would suggest you use the POD HD500 XLR outs to the FOH and the 1/4" outs for the Marshall amps into the FX return ONLY.  Do not plug the HD500 into the front of any of the Marshal amps.  This will cause you for problems, headaches, and poor sound.  Every Marshall amp since the 1980s have had an FX loop on the back of the head. If you are getting Marshall heads that do not even have FX loops, then they must being using amp about 30 years old.

Setting up your HD500....I recommend using FULL  patches with cabs/mics you like the sound of, then run the HD500 in Studio/Direct output mode. from ALL outputs.  This will give you the best possible sound on stage by just using the Marshall power amp section into the cabs, and it's the correct setting to go direct from the HD500 into the FOH console.  Unless the PA person does something very weird, you should sound nearly identical on stage as you should coming out of the PA.

A bit more non-typical set up would be to use ONLY the XLR out to the FOH console and then have a separate return from your channel PRE EQ and run THAT SIGNAL into your Marshall FX Return.  Then you know 100% certain that the sound you are hearing from the cabs on stage is the sound coming out of the PA.  This just means that the FOH console has to have and Aux Send that can be used ONLY for your guitar sound.  And I'm not even considering what mix you get in your stage monitors.  Literally you would not even need to use the amp on stage at all if the monitor system is flexible and good enough.

Another version of this would be to use an unbalanced SPLITTER/DI box to cover your signal to the console and have the same signal then brought down to an unbalanced 1/4" jack for the FX Return.  This is ONLY if you are not able to get both the 1/4" jack & XLR jack from the HD500 to send out the exact same signal.  This should be only an option IF you can't get the POD HD to work the way it was designed and how I described previously.

Anyway - the point of all this is that your situation should not have to be complicated.  IF you never use any acoustic guitars running through your HD500, you shouldn't need to do anything more than plug the POD HD into the FX Return on the back of the Marshall head and have the PA person just mike up the cabinet - just like in the olden days when none of this tech gear existed.

But these are all the ways I know as a FOH PA person for over 30 years.  I have used at one time or another all of these suggestions working with other bands using Line 6 gear or as a guitarist on stage working with a PA person who doesn't have enough inputs or outputs, etc.

Take care,

Neal