Hi, Barry;
I remember that till the HP48, all arguments possible in an array are numbers, even complex (not sure if bunaries are allowed as well... I think not). I don't recall having algebraics as array "arguments".
anyway, you can arrange your expressions in lists and disassemble/reassemble: turn lists into arrays. If I am not wrogn, the HP49G+ has some new features for this.
Also, having the expressions organized in lists, like
{{cos(x) sin(x)}{sin(x) cos(x)}}
allows you to use a handy command: DOLISTS (also DOSUBS may help, depending on what you want).
I guess SYS RPL guys may have other possibilities. I'm starting to read about SYS RPL, and there are many interesting things there... It's a new world.
Hope you succeed. If not, post again.
Cheers.
Luiz (Brazil)