Update:
I found a document about the quarantine rules at the website of the Bavarian health ministry (
http://www.stmgp.bayern.de/wp-conten..._eqv_en_bf.pdf), wherein is stated:
"Do I have to go into quarantine if I am only travelling through Bavaria? If you are just travelling through Bavaria, you do not have to go into quarantine, but you do have to leave the Free State of Bavaria on an immediate route. A short rest for safety reasons is permitted."
This suggests to me that it may be possible to stay in an airport hotel. I followed up with a telephone call to the German Federal Police at Munich Airport, explaining my situation to them. The person I spoke to said that if I was continuing to Finland, the Finnish authorities would decide on quarantine measures. When asked specifically if I could stay overnight in an airport hotel in Munich, I was told that if I want to be safe, it would be best to get a Corona test in Spain prior to flying to Germany. I asked if it would be possible to get the test at Munich airport instead, and was told yes, but the results would not be ready immediately.
So it is still not entirely clear to me whether they would prevent me from exiting the airport and taking a cab to an airport hotel. I will look into Corona test possibilities in Spain.