i'd think you can do it by making it soo long that it on the end goes loop, so if you make a 24h show. 30sec show is like 1 hour long on the rct3 timer. So make it 12 min long and you have a loop...
ps. I'm trying this out on one of my coasters now...
EDIT:
Ok, on my calculations is 24 sec show, about 1 hour in the game:
hours in game | 1 | 2 | 3 | 4 | 8 | 12 | 24
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seconds show | 24 | 48 | 72 | 96 | 192 | 288 | 576
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minutes show |0:24|0:48|1:12|1:36|3:12 |4:44 | 9:28