Shadowlore said:
That would be a helluva house rule, and it would be opposition to the specific example of UMD in the PHB.
The class feature of Turn undead (Channelling more broadly describes) does not say you create positive energy, it says you channel it. Emulating the class feature provides the channeling; the positive energy is around you (ind generally a part of you) and you merely provide a conduit for it.
Not Really (Although few of my players take UMD. We have such a large group & so many Arcane/Divine PC's, UMD becomes more like Disable Device, as the PC only gets to use it when confronting something new, as magic gear he would need to use UMD for tend to wind up with PC's who have it as a class feature, no roll required).
If the UMD roll in question is one not mentioned in the PHB, I give a minimum of 25 (activate blindly) +5 DC (for things that seem 'easy', but I generally give +10 or more to the DC). (Plus the 'special tools' qualifier, much like Open Lock requires some sort of lock-pick before you can use it).
As for Channeling Positive Energy. That's what the Healing Potion is for. In order for the rogue to mimic a Turn Attempt, he needs a source of Concentrated Positive Energy. He would use the Healing Potion in such a way as to mimic a Turning's Channeled Burst of Positive Energy.
Here's how I would have Ruled in this Example.
1. The character would need to make a UMD check to Emulate being a Dwarf (Assuming he wasn't one to begin with). DC 25 (PHB). You may try this till you succeed or you roll a 1.
2. A UMD check to Emulate an Alignment (If the Portal was keyed to one, the posters seems to inicate LG, which a rogue/bard probablly isn't). DC 30 (PHB). You may try this till you succeed or you roll a 1.
3. A UMD Check DC 30 (Activate Blindly's 25 +5). If this is successful, I'd tell the PC he need's to have a concentrated source/focus of Positive Energy to 'Fool' the door into believing he's just spent a Turning Attempt. My players are pretty good about this & would go for a potion, wand, scroll, etc. You may try till you succeed (or roll a one. Each attempt still requires a source of concentrated Positive Energy.
4. For flavor, I'd describe it like " You take the lit tourch, fill your mouth with so much of the healing potion you resemble a squirrel upon the Para-elmental Plane of Infinite Nuts and expel it through the lit torch. As the potion hits the tourch, the flame turns blue & a massive cloud of blue mist hits the door. Upon which it slowly opens upon its hinges.
See. Not a Real Likely scenerio, but possible. I tend to view Use Magic Device more as Emulate Macgyver. Its more fun that way.
Edit: I gues you can Re-try UMD Checks unitl you succeed or you roll a 1. Editing my above sceneriao to reflect that.