I don't think it's immoral in any way and have done the exact same thing - even when the bulk of the miles my company "purchased" from me had been earned on business travel and business expenditure on my personal CC.

I needed to go to IAD about 3 years ago and had a load of *A miles earned on AC. It was for a conference so the dates were known about 6 months ahead so a J redemption was available. I too am a company director and said to the rest of the board I would be happy to redeem these miles for a J ticket on UA and would do so if I received around 80% of the cost of a non-flexible J ticket in return. IIRC this was about £2,400 I received as it was about £3k for the ticket if I had bought it.

Rest of the board were happy - the company saved £600. I simply expensed it - so rather than a £3000 receipt from UA there was a £2,600 one from me personally.

Everyone won ^

Edit to add that when the Auditors arrived at year end they simply asked me to prove I had taken the flights. Our financial controller had anticipated this and had asked me to save the BP's as she had thought they might want to see these. All was fine once they saw it, we have a "dispensation" (I think that is what it is called) with the tax folks regarding expenses so my company didn't even have to declare it on my P11D