Hi James,
Your aluminum mast will no doubt work if you want to use it as a vertical. I home brewed and used for Portable operation some time ago a 5/8 lambda vertical for the 6m band made with copper pipe. The copper pipe radiating element was kept vertically with the help of a non-conductive (plastic) trypod and the ground plane was made with 6 wires of about 5m layed on the ground.
You can see it towards the left side of this picture in my eQSL for that portable operation back in 2009 from Aranjuez-Madrid.
From that location, I made several QSOs into EU with this antenna and the 5 watts of my FT-817 and I even crossed the Pond making a QSO with V29 in the Caribbean (Antigua & Barbuda Is.)
Using Al instead of Cu will slightly increase the resistance and the RF radiated will be slightly less, but there are so many antennas made of Al in the market performing wonderfully that you can easily figure out how negligible the difference in performance must be.
Regarding the remote station in a property you have away from the urban noise, I highly recommend it. As you know by now, I have recently set up my own remote station and although it’s still an starting one with no antenna switch and antenna rotator yet, I’m enjoying it very much as a RX/TX antenna on the 20m band and also as a RX antenna for the rest of the bands. The way I’m doing this is by having 2 antennas connected in parallel to the radio through an antenna tuner. One antenna is the TH5-DX 5 elements tribander yagi I have on top of a tower about 15m above the ground and the other antenna is an inverted vee broadband folded dipole I have with its appex high about 14m above the ground and hung from the same tower where I have the yagi.
The yagi can’t rotate at the moment and it’s fixed pointing to North-America.
The broadband dipole in that low noise environment lets me copy most or nearly all of the activations in EU when propagation conditions are OK and the yagi let’s me hear those weak signals from NorthAmerica that the dipole can’t copy.
My remote station 2 antennas in parallel are tuned for low SWR on 20m, but I can’t currently transmit on the other bands. So, for the bands other than 20m, I’m receiving in remote with those described antennas and transmitting with my endfed wire in the balcony of my rental appartment in the city and I assure you it’s working very well for me.
I fully support what Ed suggested you in a previous post: if you don’t trust setting up a complete expensive station in your remote low noise location, you can set up a RX remote antenna system (quad or delta loop is a good choice) and keep transmitting from your current noisy location. The SOTA activators will surely copy you.
Good luck and have fun.
73,
Guru