heyas;
no, you can't select 50 items in your inventory and set them ALL to the same properties all at once. however, this is my method. while still manual, i think it's the quickest way to date.
1: select all the objects you want to set perms for (in inventory or in an object's contents.)
2: right click and select properties. this will open all the object property panels in one collective window.
3: use your mouse to check/uncheck whatever properties you need to change.
4: tap ALT + LEFT CURSOR to switch to the next tab.
5: keep clicking/tapping through the stack of property windows.
NOTE: when you get to the 'bottom' of the stack, it will loop back to the top. keep an eye on the boxes so you don't start un-checking what you meant to be checking and vice-versa!
TIP: when doing this on an object's contents, close the object edit palette before you start. for some reason, if the edit window is open, it takes the properties longer to finish changing. (this is really only important if you are changing more than one property at atime. and/or are really, REALLY impatient.)
you can also use this method to rapidly scan object permissions you need to inspect for accuracy. but instead of clickng and alt-left-ing, click the inner 'close' X on each permission tab as you check it. when you get to the last one and are done, the window will close.
hope this helps!