Yes, it's all relative to the main verb - "when I saw him..."It occurs to me that we have the same issue in English. Consider the sentence "I saw him standing". Standing is a participle here, the word "stand" can mean "stand up", but when I say I am standing, the focus is not on the act of standing up at all. The tense says that at the time I saw him, he was standing. I can say, "10 years ago, I saw him standing behind the doughnut shop", which indicates that he was standing at the time I saw him.
Likewise, in Rev. 14:1, it's all relative to the main verb, even though, as Barry indicated, the participle rendered "standing" in English is used as an adjective to describe the Lamb - he is the "standing" Lamb. Even though that is the case, the participle still is relative to the main verb - I find that very interesting. So, as you've pointed out, there doesn't exist any emphasis on the act of standing, only upon the state that resulted from his standing up - namely, that of "standing".
So John did not see the Lamb stand up - he only saw him in the state or condition of "standing". Why did he not simply use the present participle, then, instead of the perfect? Perhaps there is no reason other than it just isn't done that way in Greek?
Bottom line seems to be that there isn't any reason, related to any precise meaning John may have wanted to convey, for his selection of the perfect tense for the participle.