Firstly, be sure fermentation has completely finished before bottling. 1.029 is a high FG - from a SG of 1.090, that's about 8% abv. I would rouse the yeast, maybe even add additional yeast.
You can use potassium sorbate to halt the fermentation, but at a relatively low 8% abv, you will need a lot of it, more than the 0.18g/l taste threshold. Better to let the yeast consume the available sugars to increase the abv a little, and then add the sorbate.
I can't recommend the baking soda route, since I've not tried it. But I thought I'd try to work out how much you might need and what this might do to the beverage.
Depending upon how much carbonation you want, the target carbonation is around 2 to 3.6 volumes - representing the scale from lightly carbed beer to soda. Assuming the hard lemonade has been fermenting under an airlock, you'll end up with around 1 volume of CO2 already dissolved, so we need to produce an additional 1-2.6 volumes of CO2.
Baking soda and citric acid react in water:
3NaHCO3(aq) + C6H8O7(aq) --> 3CO2(g) + 3H2O + Na3C6H5O7
That's 3 moles of baking soda, plus 1 mole of citric acid produces 3 moles of CO2, 3 moles of water and 1 mole of trisodium citrate. Trisodium citrate is food additive E331, and is common in citric soft drinks. It has a tart, slightly salty taste.
So, how much carbonation will that give? 1 mole of CO2 produces 24.4 liters of gas at STP. To produce 1 volume of CO2 in a 19 liter (5 gallon) batch, we then need to produce 0.78 mol of CO2. The reaction formula above shows we need 2.33 mol baking soda, and 0.78 mol citric acid to produce 0.78 mol of CO2. Baking soda has a molar mass of 84, while citric acid is 192, so that's 195g of baking soda and 150g of citric acid to produce 1 volume of CO2. For higher CO2 volumes, simply multiply the additional volume required with these values.
For example, if you were going to carbonate to soda levels (3.5vols co2), then you'd need to produce an additional 2.5 vols CO2, which requires 2.5*195=487g of baking soda and 2.5*150=375g of citric acid.
Trisodium Citrate has an RDA of 2.5g per day. Assuming you bottle into say 50 or so 12oz bottles and a paltry consumption of one bottle per day, then the maximum carbonation level you can achieve is:
2.5 x 50 = 75g of produced sodium citrate, or 75g/258g/mol = 0.29 mol, which means we can only produce 0.29 mol of CO2 or 0.29/0.78 = 0.37
additional volumes of CO2, giving a final carbonation of 1.37 volumes at STP.
To sum up: using the RDA of 2.5g for the trisodium citrate, we can produce an additional 0.37 volumes of CO2, to give a total of around 1.37 volumes in the beverage. This would require 73g of baking soda in the entire batch, which seems like a lot of baking soda for not a lot of CO2.
