On the East Coast Main Line at Connington headspans have been replaced with booms (hope that's the correct terms) because of dewirings, so I would assume similar work would be feasible on the MML. There's picture here showing the masts looking oddly tall:
https://twitter.com/networkrail/status/946395340940308480
I'm also curious to know how far north this work needs to start. From a standing start at St Pancras I am guessing 125 mph isn't possible until Mill Hill, if not further out, so I would assume there's no need for works where lower speeds apply? (There may of course be supply issues rather than tensionning.)
Not far off there! The Conington headspan portal conversions were indeed due to dewirements - but also to act as lateral braces in high winds (which that short area of route is prone to). Indeed, one such boom has already gone in on the MML at Borehamwood Neutral Section, to support a quartet of Earthed MIR Cantilever assemblies.
The only problem with the Conington arrangement, as it stands, is the tensioning arrangements - there isn't a definitive Mid-Point Anchor yet (it's still all pulleys, links and clamps), the catenary is still bridled (again, the pulleys), and at overlaps, the Fast lines' OLE still uses Tail Wires to anchor at a structure beyond the structure anchoring the Slow lines' OLE. Plus, NR haven't found a solution regarding those headspans where Booster Transformers were formerly located - they're much trickier to portalise.
EDIT: I've been having a look at the Sectional Appendix for the MML; 110mph begins on the Fasts at 5m 30ch (north of Cricklewood), although this may change when Brent Cross West gets built. This drops to 105mph through Hendon (6m 40ch to 7m 33ch), whereupon 110mph resumes until 12m 70ch (Up Fast)/12m 71ch (Down Fast). North of here, 125mph is the top speed limit for HSTs (which includes 222s, and will doubtless include 180s and 810s); additionally, the speed limit for non-high speed stock is 100mph through Elstree Tunnel.
The 110/HST125 differential continues until 18m 38ch (around Napsbury), where it becomes 100/HST110 until 19m 21ch (Up Fast)/19m 24ch (Dn Fast), where a blanket 100mph limit is imposed through St Albans City.
The linespeed of the Fasts, which change throughout the route north of St Albans, never drops below 100mph (except through Luton), while the Slows' linespeed never exceeds 90mph.