olkknotty - thanks for posting the photos as they help a lot
Do you know what/who made your Zouave?
As Steve states . . . it depends on the maker as to the lock geometry and if it is such that half-cock clears the nipple enough to allow you to cap on half cock.
My Zouave was made in the early 1960s - it is not marked as to maker. I have set it side by side to an original 1863 Remington though and mine is as about as close as you can get for a reproduction.
On your Zouave - I see two things that contribute to the hammer not clearing the nipple enough on half-cock so that you can cap it.
First, the tumbler notch for the half-cock is positioned so that the hammer obviously falls too close to the nipple on half cock. It appears that it was made this way by the manufacturer - in essence, it is what it is.
Second - the barrel bolster is higher - mine is lower than yours which only means that the nipple sets lower and this also contributes to more space at the half cock position.
These observations in no way mean that your Zouave is inferior in any way or there is a problem with it. It is just the "nature of the beast" as some of the repros were made this way. Your Zouave should function just fine.
Steve also goes on to make some good points. As long as your full cock position and trigger pull are "heavy" enough that there is no chance of the hammer falling (a good way to test is to put it on full cock - unloaded and uncapped of course - and press against the back of the hammer spur to see if it will slip and fall - from the looks of your tumbler, it won't be a problem on yours). In shooting NSSA or range shooting, I capped at full cock and that shouldn't present a problem as Steve and others have and will point out.
If you are going to hunt with it, you will have to full cock it, place your cap on and then very carefully lower the hammer in to the half cock position.
I have a repro 1855 rifled musket (made by Armi-Sport if I remember correctly?). The lock on that is nothing like an original - it has a completely different feel and the geometry of the tumbler is also different from an original. I ended up replacing the tumbler on it with a tumbler out of a 45-70 that fit it and had the full cock and half cock notches in a better location than what came with the rifle.
I would just go ahead and shoot your Zouave and enjoy it. If down the road, the half-cock position really bugs you, you could have someone like Lodgewood Mfg. or S & S replace the tumbler and tune your lock so that the half cock allowed you to cap. They do a lot of custom work for NSSA shooters but you really should be fine as long as you are careful when lowering your hammer to half cock for hunting, etc. At the range, you can cap at full cock - keep your finger off the trigger until ready to shoot and then just go ahead when you're ready.
As always, just be safe and keep your rifle pointed in a safe direction all of the time. You'll be fine and you'll enjoy shooting that Zouave!