I don't know about legal/accounting issues...(I don't think there would be because companies do this all the time) but I would be wary.
My first job out of college was a travel job and the company I worked for required us to use Amex. Fine, no problem. The problem (I didn't realize at the time or I wouldn't have gone along) was that it didn't matter if the charges were all company related, the responsibility and liability was 100% on the employee. The company would cut a check to the employee (who would then pay the Amex bill). When I left, I cut up the Amex card and verified that I had a zero balance.
Fast forward a year...the company suddenly goes out of business and several of my friends who still worked there were shocked when Amex told them it didn't matter that the company had gone bankrupt...they still owed the $$. Some of these guys would rack up $10,000 in a month due to all of the traveling. Not fun...
I think I understand what you want to do (pay the $15,000 on your card, get the rewards/points then use the company reimbursement to pay it off...correct?)
I would want to make sure the company has the ability to pay, would pay in a timely manner, don't have an internal policy somewhere against this (that is suddenly "discovered" AFTER you've paid), etc, etc.
Personally, I wouldn't. But I am extremely conservative.