Interesting question!(Though personally I've no idea why you want to make a decorative article out of lithium, but I guess it's best not ask)
You did consider using the process we know as electroplating; yes, it's one of the common, commercial methods of 'passivating' (rendering inert) a lot of metals that are prone to corrosion, like iron for example (Galvanization). However electroplating is something that could never work out with lithium, here's why...
(The above example, courtesy Google Images, suggests the use of copper as the electroplating metal, but since I can't seem to find a more generalized diagram so we'll just go with this)
A quick crash course on the set-up for electroplating:
Your cathode is the metal to be electroplated, while your anode is the metal (copper here) that is used to electroplate whatever you've hooked up as the cathode. Both the anode and cathode are immersed in an aqueous solution of a salt of the metal used as the anode (here, it's copper sulfate).
Clear with that? Good, now here's the catch...
You need to put lithium in an aqueous solution, that is, a solution that uses water as the solvent. In your question you voice concerns over lithium's reactivity towards stuff you typically come across in your surroundings (oxygen and carbon dioxide in the air, moisture/humidity, etc), so I take it that you are well aware of what happens when you dump a large, fresh piece of lithium in water...
So yeah, don't even think about electroplating this.
The making itself aside, what can I do to protect it long-term from reacting with the atmospheric water and gases?
Well, there are a couple of other methods you could use:
1) Coating it in wax.
Coating the lithium in a thin, uniform layer of wax (candle wax, beeswax, furniture wax or any sort of wax that you can easily get your hands on, and remains a solid at room temperatures) would do a pretty decent job of protecting lithium from your ordinary household environment. It shields the metal from both atmospheric gases and moisture. Heck you could even dump the whole thing in water and it wouldn't go off in your face. But evenly coating a "decorative object" made out of lithium with wax while still being able to see the object clearly under the wax will be a bit tricky...even more so, if this "decoration" of yours has any really intricate patterns.
This is where we move on to the next method,
2) Immersing it in a jar full of mineral/paraffin oil
This is far simpler than coating the thing in wax. Get an airtight, transparent jar, fill it all the way to the brim with mineral oil, slowly immerse the object in the jar so you don't trap any air bubbles, screw on the lid tightly and there you have it! True, a major 'disadvantage' here, is that you can't take the object in your hands and examine it closer as and when you wish (which wasn't the case in the waxing method), but maybe a shiny lithium decoration floating around in a jar might make up for this aesthetically.
Which of the two methods you use, is entirely at your discretion, however, a word of caution:
Lithium metal is a fire risk! Do handle it with care. Have necessary safety arrangements in place.