If you don't mind giving up your 12 on the cassette, you could buy a 13-25 and combine the loose cogs on the 13-25 with the carrier cogs on the 12-27 for one custom cassette, and create another cassette out of the other two halves like this:

13-25: 13,14,15,16,17,19,21,23,25
12-27: 12,13,14,15,17,19,21,24,27
12-25: 12,13,14,15,17,19,21,23,25




Custom 1 : 13,14,15,16,17,19,21,24,27

Custom 2: 12,13,14,15,17,19,21,23,25

So Custom 1 has your 16t and 27t, and Custom 2 is exactly like a stock 12-25 you could use on another wheel.