You can. People calculate the carbon footprint of big complicated things. But you end up balancing accuracy with effort.

You might be able to calculate some stuff using a bottom-up approach. Like you said, direct emissions wouldn't be too hard. Travel emissions would be a bit trickier, but maybe you have location data of ticket pruchasers that can be used or maybe you make some assumptions around travel distance and travel mode (assumptions could be based on a surveyed sample of attendees or something else). Similar with logistics. Some data might be available, some you might to assume. For all the disposable stuff, you might be able to get total volume or weight purchased and then use some general emission factor for plastic production/shipping. For things that are particularly challenging bottom-up, you can use top-down economic input/output models (e.g. $X of spending in this sector results in Y tonnes of GHG emissions).

TL;DR: Yes, it is possible but for things that complicated there are tradeoffs between accuracy and effort to quantify.