What is the transmembrane 'Positive-Inside Rule' nowadays? Has the definition changed over time?
I think you have misunderstood the "inside" part of the "positive-inside rule". Perhaps because "inside" is indeed an imprecise term (but now it is history and cannot be changed ;) ). In order to understand it a bit better it helps to think about the topology of the membrane. During synthesis most membrane proteins (ignoring peroxisomal and mitochondrial proteins, which are a whole other topic) are inserted into the ER membrane by the Sec translocon. The parts of the membrane protein that are exposed to the lumen are matured by the ER and Golgi apparatus to be presented on the extracellular surface. Secretory proteins (those that will ultimately be exported from the cell) are made in similar way except they pass entirely through the translocon and end up in the ER lumen. Therefore, the lumen of the ER (and consequently the lumen of other endomembrane compartments) are topologically consistent with the extracellular space, "outside", and not the "inside" of the cell. Perhaps is is easiest to see in a picture (borrowed from here: http://www.bioon.com/book/biology/mboc/chapter12/figure12-4.gif)
Coloured in pink are compartments whose topology is consistent with the "outside" and coloured grey are compartments whose topology is consistent with the "inside". So, for membrane proteins found in the endosome, for example, amino acid residues in the lumen are actually "outside" not "inside". It is true that this may not be immediately clear to those not steeped deeply in the field. As the initial studies used bacteria, which do not have membrane-enclosed subcellular compartments, "inside" was simple and made sense. Perhaps "positive-in-the-cytoplasm rule" would have been more accurate. Hope this helps!