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Is there any reason why I cannot use the DAC on the U3 as the supply voltage for a 350 Ohm bridge. With a DAC set to 5V, this should double the analog input to the U3?
You could use the DAC outputs. At best, this could provide twice the measurement resolution. However, the DAC outputs are less accurate and noisier than the Vref. The specified accuracy of the DAC outputs is around 5% full scale (5V*0.05) whereas the Vref is around 0.2% full scale (2.5V*0.0025). There is also additional noise and error as you drive closer to 5V on the DAC outputs. So depending on your setup, you may see better measurement resolution from the DAC excitation, or the extra noise and lower accuracy could offset the benefits. Also note that the typical max DAC output is 4.95V and dependent on the value of VS. See the datasheet appendix for further information:
Another thing to consider is that the 2.5V Vref could be more useful than a higher voltage excitation depending on the sensitivity/voltage range you are working with. For example, the 2.5V reference could come close to maximizing range and resolution from an InAmp/LV input setup if you have a load cell with 2mV/V sensitivity and you need to be able to measure full scale in both directions (tension and compression).
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You could use the DAC outputs. At best, this could provide twice the measurement resolution. However, the DAC outputs are less accurate and noisier than the Vref. The specified accuracy of the DAC outputs is around 5% full scale (5V*0.05) whereas the Vref is around 0.2% full scale (2.5V*0.0025). There is also additional noise and error as you drive closer to 5V on the DAC outputs. So depending on your setup, you may see better measurement resolution from the DAC excitation, or the extra noise and lower accuracy could offset the benefits. Also note that the typical max DAC output is 4.95V and dependent on the value of VS. See the datasheet appendix for further information:
https://labjack.com/support/datasheets/u3/appendix-a
Another thing to consider is that the 2.5V Vref could be more useful than a higher voltage excitation depending on the sensitivity/voltage range you are working with. For example, the 2.5V reference could come close to maximizing range and resolution from an InAmp/LV input setup if you have a load cell with 2mV/V sensitivity and you need to be able to measure full scale in both directions (tension and compression).