Hi. Sorry I didn't get back to you on this post until today.
To (partially) answer your question, I thought I would do a quick survey of all of the teams in this year's PC tourney that made it to the playoffs in events 1-5. Out of 196 playoff teams in events 1-5, the only 3 teams that played in Nationals were these ones:
Nationals
http://onlinegames.strat-o-matic.com/team/1110534http://onlinegames.strat-o-matic.com/team/1112239http://onlinegames.strat-o-matic.com/team/1113461You could have a look at these to see if there are any common themes to them, from your perspective.
I also made notes of the following teams that are in "somewhat" similar ballparks, in terms of a middle of the road ballpark home run chance (say between 1-7 and 1-14, but excluding Rogers due to its slant in favour of RH hitters). This is what I found:
Minute Maid
http://onlinegames.strat-o-matic.com/team/1114164http://onlinegames.strat-o-matic.com/team/1114466Turner
http://onlinegames.strat-o-matic.com/team/1112652http://onlinegames.strat-o-matic.com/team/1114081Citi
http://onlinegames.strat-o-matic.com/team/1114724Wrigley
http://onlinegames.strat-o-matic.com/team/1107156Chase
http://onlinegames.strat-o-matic.com/team/1106947http://onlinegames.strat-o-matic.com/team/1108408http://onlinegames.strat-o-matic.com/team/1110174http://onlinegames.strat-o-matic.com/team/1111314http://onlinegames.strat-o-matic.com/team/1111214http://onlinegames.strat-o-matic.com/team/1114071Comerica
http://onlinegames.strat-o-matic.com/team/1110556Dodger
http://onlinegames.strat-o-matic.com/team/1112664Camden
http://onlinegames.strat-o-matic.com/team/1106839http://onlinegames.strat-o-matic.com/team/1106748http://onlinegames.strat-o-matic.com/team/1108303http://onlinegames.strat-o-matic.com/team/1110366http://onlinegames.strat-o-matic.com/team/1110444http://onlinegames.strat-o-matic.com/team/1110663http://onlinegames.strat-o-matic.com/team/1112747http://onlinegames.strat-o-matic.com/team/1114089http://onlinegames.strat-o-matic.com/team/1114888http://onlinegames.strat-o-matic.com/team/1114088http://onlinegames.strat-o-matic.com/team/1115077(Note, I excluded one "hit" on this list, because the record was just 81-81, even though the team made the playoffs).
Clearly, the big favorites for successful, moderate ballparks were Chase and Camden.
Camden is interesting, because it is getting up there towards the home run parks, at 1-14 ballpark home run chance. Homerun parks were *very* popular as PC tourney playoff teams. So Camdens popularity and success may be more attributable to being an "almost a home run park".
Chase, on the other hand, is fairly similar to Nationals. So the Chase teams (and other listed above) might be good comparisons to Nationals, for further research by yourself.
Here's what really strikes me (!) - that, if I exclude Camden (for the reasons that I just stated), that only 17 out of 196 playoff teams used a "middle of the road" park!!! That's a very low ratio!!!
I did one more thing - I quickly clicked into each team to see what the team ERA was. What I found was that about 1/2 of the teams in the upper section above had a team ERA under 4, and about half were over 4. Most of them were around 4.00. But once you got down to the Chase and Camden teams, most of the team ERA's were over 4 - especially for the Camden teams. The Camden teams looked like they were built more as hitter/homerun teams. So the Camden ones may or may not be useful, depending on your own thought as to how best to use Nationals. Team ERA over 4 might suggest a "hitter's park" strategy was popular for these ballparks. I wonder how successful that strategy was, though, apart from Camden (where it seemed to be successful).
Caveat - I haven't tried to do a full survey of all attempted uses of these parks, to get "success" ratios, in terms of making the playoffs. My guess is that the success ration might be a little lower than either (a) pitcher parks (b) slugger parks or (c) unbalanced parks, which were very popular in the PC tournament. The issue is to find the answer to the question, "What will be this team's advantage compared to other teams in its division or its league?".
I'd be interested to see if any other managers have some observations on all of that!