Must echo the comments of the grumpy ol' German blacksmith. Coal can be safely burned, just as any other fuel can be safely burned in a stove based tent, if adequate precautions are taken. The danger of CO or CO2 is present regardless of the flame/fuel type, if adequate ventilation is not maintained.
In using wall tents, we have pitched near seams of coal, and simply hauled buckets of coal to the tent. Low quality to be certain, but the volume of coal burnt was very small in comparison to the wood required. A close friend weathered 10 days on the eastern slope of the Rocky Mts by burning coal, during this time the daily high never surpassed -45C . They read their books twice and rarely ventured beyond the tent other than to gather coal, bring snow for melting, and to answer the call of nature. They ate/slept/lived with a coal fire for 10 days. Afterwards they chided me on not making the time to join them. The intention had been to put up a number of new ice climbing routes in a remote valley, all from the comfort of a hot tent. They didn't climb a single pitch over the course of their stay. The ice was too brittle due to the cold.
Satisfy your curiousity, measure the CO levels by setting up in the back garden. Likely you will find that a very small amount of ventilation will keep everything on a healthy level. Post your results and let us risktakers know how much danger we were truly in. Unless your tents are much more airtight than the ones in our neck of the woods, my suspicion is that you will be just fine.....and warm with a coal powered stove.
Good Luck.