Altium, it is possible to mirror the silk screen?
Yes you can mirror text very easily, or more correctly swap the layer it is on.

Place the components as you would per your layout. Then on the top silk (I am assumng you are placing on the top layer), select the designator of the component which you would like to mirror. The designator should float with your cursor allowing you top re-position it wherever you like. While moving the designator, press "L" on your keyboard. this will assign it to the bottom silk layer and will automatically mirror it for you.

Make sure to preview the deisgnators to ensure they are readable. You can Flip the board by pressing "V", "B" with the bottom silk visible and the text should be readable. If it is not, you may mirror it by selecting the designator and pressing "X" or "Y" to apply the appropriate mirror.

You can select all the designators using the PCB filter functions however when you swap layers with everything selected, the entire selection will be transposed requiring manual relocation anyway. The only way to do it is on a one by one basis. I hope you don't have too many components.

Be sure to generate a bottom silk gerber file if you are exporting them.