So, you probably can just rotate the tensioner and get it through the inspection hole without too much issue, it will be much easier installing it.
The main thing you'll be fighting if you do that is the tension on the actual chains themselves. Even at TDC it is possible for the cams to still want to roll some and skip timing when you take tension off the guide; though at TDC it should have the least amount of chance to do that. I mention that because on the 5.7L V8's when you're trying to time the chains after resealing the camshaft housings, Bank 1 likes to be very touchy and will sometimes roll on it's own just breathing on it. Just pushing on the guide isn't going to be enough (that's why the tensioner is a ratcheting design unlike the two tensioners used for the chains between the Intake and Exhaust Camshafts, so that even if it loses oil pressure it shouldn't be able to be pushed physically back far enough to let the chain slip, at least in theory).
Normally, this slipping isn't a super big deal because you have the timing cover off and valve covers off so you can just throw a wrench on the camshaft to hold it in place and wiggle it as needed so it doesn't spin on you. The thing is, I can't remember which cam wants to roll on you on a GR motor; I maybe do a timing cover or cylinder head gasket on one every once in a blue moon (we have more 5.7L guys buying timing cover/camshaft housing reseals than we do V6 owners). So you'd also want to pull one or both valve covers off so you can either make a tool to "lock" the camshafts at TDC (Toyota doesn't have a tool for this like Ford, it isn't something we ever have to do) or find the one that wants to roll on you and have someone hold it with a wrench. To give you an idea what I mean, here's a better picture of where the No.1 Timing Chain Tensioner lives on the front of the block along with a whole view of the front of the engine with the cover removed (I think this picture is a 2GR-FE, but the way the chains, guides, and tensioners are set-up is pretty much identical).
One more thing I will mention is that I wouldn't bother with replacing just the tensioner. Because chain stretch is an actual thing, usually by the time you hear the chain slapping, the guides will also have some excessive wear (they also get very brittle at high age/mileage when they dry out, I don't know exactly why but I have had the plastic shatter on a few over the years from sitting for a day or two not bathed in oil). This is why I always add an additional recommendation to vehicles with a lot of years and/or miles (basically anything over 10yr/150k) to replace the timing chain, guides, tensioners, and at least a crank sprocket when I recommend doing any work that will expose the timing chain or require it's removal (It's usually either just parts if I am already taking them off, or at least an hour labour if I have to take everything off and re-time it.)
In any case, I don't usually recommend people replace their timing chain components unless there's an issue with them and/or we're already tearing down the engine that far and the vehicle is older/higher mileage. But it doesn't hurt to check your timing chain and timing if you have the valve covers off to give you a rough idea if they need to be replaced.
Here's a good example of what I mean by timing chain stretch, I stumbled across this guy's issues with his FJ Cruiser (oddly enough, we have seen a lot of stretched chains specifically around the 2007 model year on mainly FJ Cruisers, but some 4Runners, and Tacomas; not sure if it was like a bad batch of chains or if it's just a coincidence?) and a few pictures of what the chains look like.
Stretched timing chain | Toyota FJ Cruiser Forum
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Hopefully that isn't too much rambling for you, I just was trying to give you as much information as I could for your question in case you did want to try it and also share why I don't think I'd bother with it at this time.
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